Trade: AbstractSceneConverter::begin*() should have a way to fail.

Initially I thought there was no reason for this to fail, but then
realized AnySceneConverter would need it. And also any other plugin
relying on something external that might fail during initialization --
there's no other moment after plugin instantiation where it could
signalize a failure, and deferring that to the first add() call,
whichever would it be, is really not a sane idea.

src/Magnum/Trade/AbstractSceneConverter.cpp

@@ -301,22 +301,28 @@ void AbstractSceneConverter::abort() {
 
 void AbstractSceneConverter::doAbort() {}
 
-void AbstractSceneConverter::begin() {
+bool AbstractSceneConverter::begin() {
     if(_state) abort();
 
     _state.emplace(State::Type::Convert);
 
     if(features() >= SceneConverterFeature::ConvertMultiple) {
-        doBegin();
+        if(!doBegin()) {
+            _state = {};
+            return false;
+        }
+
+        return true;
 
     } else if(features() & SceneConverterFeature::ConvertMesh) {
         /* Actual operation performed in doAdd(const MeshData&) */
+        return true;
 
-    } else CORRADE_ASSERT_UNREACHABLE("Trade::AbstractSceneConverter::begin(): feature not supported", );
+    } else CORRADE_ASSERT_UNREACHABLE("Trade::AbstractSceneConverter::begin(): feature not supported", {});
 }
 
-void AbstractSceneConverter::doBegin() {
-    CORRADE_ASSERT_UNREACHABLE("Trade::AbstractSceneConverter::begin(): feature advertised but not implemented", );
+bool AbstractSceneConverter::doBegin() {
+    CORRADE_ASSERT_UNREACHABLE("Trade::AbstractSceneConverter::begin(): feature advertised but not implemented", {});
 }
 
 Containers::Pointer<AbstractImporter> AbstractSceneConverter::end() {
@@ -378,22 +384,28 @@ Containers::Pointer<AbstractImporter> AbstractSceneConverter::doEnd() {
     CORRADE_ASSERT_UNREACHABLE("Trade::AbstractSceneConverter::end(): feature advertised but not implemented", {});
 }
 
-void AbstractSceneConverter::beginData() {
+bool AbstractSceneConverter::beginData() {
     if(_state) abort();
 
     _state.emplace(State::Type::ConvertToData);
 
     if(features() >= SceneConverterFeature::ConvertMultipleToData) {
-        doBeginData();
+        if(!doBeginData()) {
+            _state = {};
+            return false;
+        }
+
+        return true;
 
     } else if(features() >= SceneConverterFeature::ConvertMeshToData) {
         /* Actual operation performed in doAdd(const MeshData&) */
+        return true;
 
-    } else CORRADE_ASSERT_UNREACHABLE("Trade::AbstractSceneConverter::beginData(): feature not supported", );
+    } else CORRADE_ASSERT_UNREACHABLE("Trade::AbstractSceneConverter::beginData(): feature not supported", {});
 }
 
-void AbstractSceneConverter::doBeginData() {
-    CORRADE_ASSERT_UNREACHABLE("Trade::AbstractSceneConverter::beginData(): feature advertised but not implemented", );
+bool AbstractSceneConverter::doBeginData() {
+    CORRADE_ASSERT_UNREACHABLE("Trade::AbstractSceneConverter::beginData(): feature advertised but not implemented", {});
 }
 
 Containers::Optional<Containers::Array<char>> AbstractSceneConverter::endData() {
@@ -429,26 +441,32 @@ Containers::Optional<Containers::Array<char>> AbstractSceneConverter::doEndData(
     CORRADE_ASSERT_UNREACHABLE("Trade::AbstractSceneConverter::endData(): feature advertised but not implemented", {});
 }
 
-void AbstractSceneConverter::beginFile(const Containers::StringView filename) {
+bool AbstractSceneConverter::beginFile(const Containers::StringView filename) {
     if(_state) abort();
 
     _state.emplace(State::Type::ConvertToFile);
     _state->filename = Containers::String::nullTerminatedGlobalView(filename);
 
     if(features() >= SceneConverterFeature::ConvertMultipleToFile) {
-        doBeginFile(_state->filename);
+        if(!doBeginFile(_state->filename)) {
+            _state = {};
+            return false;
+        }
+
+        return true;
 
     } else if(features() >= SceneConverterFeature::ConvertMeshToFile) {
         /* Actual operation performed in doAdd(const MeshData&) */
+        return true;
 
-    } else CORRADE_ASSERT_UNREACHABLE("Trade::AbstractSceneConverter::beginFile(): feature not supported", );
+    } else CORRADE_ASSERT_UNREACHABLE("Trade::AbstractSceneConverter::beginFile(): feature not supported", {});
 }
 
-void AbstractSceneConverter::doBeginFile(Containers::StringView) {
+bool AbstractSceneConverter::doBeginFile(Containers::StringView) {
     CORRADE_ASSERT(features() >= SceneConverterFeature::ConvertMultipleToData,
-        "Trade::AbstractSceneConverter::beginFile(): feature advertised but not implemented", );
+        "Trade::AbstractSceneConverter::beginFile(): feature advertised but not implemented", {});
 
-    doBeginData();
+    return doBeginData();
 }
 
 bool AbstractSceneConverter::endFile() {

src/Magnum/Trade/AbstractSceneConverter.h

@@ -753,7 +753,8 @@ class MAGNUM_TRADE_EXPORT AbstractSceneConverter: public PluginManager::Abstract
          * If a conversion is currently in progress, calls @ref abort() first.
          * The converted output of data supplied via various @ref add() and
          * `set*()` APIs is returned via an importer instance upon calling
-         * @ref end().
+         * @ref end(). On failure prints a message to @relativeref{Magnum,Error}
+         * and returns @cpp false @ce.
          *
          * Expects that @ref SceneConverterFeature::ConvertMultiple is
          * supported. If not and @ref SceneConverterFeature::ConvertMesh is
@@ -763,7 +764,7 @@ class MAGNUM_TRADE_EXPORT AbstractSceneConverter: public PluginManager::Abstract
          * @ref end().
          * @see @ref features(), @ref beginData(), @ref beginFile()
          */
-        void begin();
+        bool begin();
 
         /**
          * @brief End converting a scene
@@ -806,7 +807,9 @@ class MAGNUM_TRADE_EXPORT AbstractSceneConverter: public PluginManager::Abstract
          *
          * If a conversion is currently in progress, calls @ref abort() first.
          * The converted output of data supplied via various @ref add() and
-         * `set*()` APIs is returned upon calling @ref endData().
+         * `set*()` APIs is returned upon calling @ref endData(). On failure
+         * prints a message to @relativeref{Magnum,Error} and returns
+         * @cpp false @ce.
          *
          * Expects that @ref SceneConverterFeature::ConvertMultipleToData is
          * supported. If not and @ref SceneConverterFeature::ConvertMeshToData
@@ -816,7 +819,7 @@ class MAGNUM_TRADE_EXPORT AbstractSceneConverter: public PluginManager::Abstract
          * @ref endData().
          * @see @ref features(), @ref begin(), @ref beginFile()
          */
-        void beginData();
+        bool beginData();
 
         /**
          * @brief End converting a scene to raw data
@@ -842,7 +845,9 @@ class MAGNUM_TRADE_EXPORT AbstractSceneConverter: public PluginManager::Abstract
          *
          * If a conversion is currently in progress, calls @ref abort() first.
          * The converted output of data supplied via various @ref add() and
-         * `set*()` APIs is returned upon calling @ref endFile().
+         * `set*()` APIs is returned upon calling @ref endFile(). On failure
+         * prints a message to @relativeref{Magnum,Error} and returns
+         * @cpp false @ce.
          *
          * Expects that @ref SceneConverterFeature::ConvertMultipleToFile is
          * supported. If not and @ref SceneConverterFeature::ConvertMeshToFile
@@ -852,7 +857,7 @@ class MAGNUM_TRADE_EXPORT AbstractSceneConverter: public PluginManager::Abstract
          * return the result from @ref endFile().
          * @see @ref features(), @ref begin(), @ref beginData()
          */
-        void beginFile(Containers::StringView filename);
+        bool beginFile(Containers::StringView filename);
 
         /**
          * @brief End converting a scene to raw data
@@ -1757,7 +1762,7 @@ class MAGNUM_TRADE_EXPORT AbstractSceneConverter: public PluginManager::Abstract
          * @relativeref{Corrade,Containers::StringViewFlag::NullTerminated},
          * however.
          */
-        virtual void doBeginFile(Containers::StringView filename);
+        virtual bool doBeginFile(Containers::StringView filename);
 
         /**
          * @brief Implementation for @ref endFile()
@@ -1832,7 +1837,7 @@ class MAGNUM_TRADE_EXPORT AbstractSceneConverter: public PluginManager::Abstract
          * @brief Implementation for @ref begin()
          * @m_since_latest
          */
-        virtual void doBegin();
+        virtual bool doBegin();
 
         /**
          * @brief Implementation for @ref end()
@@ -1847,7 +1852,7 @@ class MAGNUM_TRADE_EXPORT AbstractSceneConverter: public PluginManager::Abstract
          * @brief Implementation for @ref beginData()
          * @m_since_latest
          */
-        virtual void doBeginData();
+        virtual bool doBeginData();
 
         /**
          * @brief Implementation for @ref endData()