imageconverter: print available plugins on importer/converter load error.

At first I wanted to add --importers / --converters options, but that
didn't really work because the argument parser complained that
input/output is not set.

doc/changelog.dox

@@ -36,7 +36,15 @@ See also:
 -   @subpage changelog-extras "Extras changelog"
 -   @subpage changelog-examples "Examples changelog"
 
-@anchor changelog-latest
+@section changelog-latest Changes since 2019.10
+
+@subsection changelog-latest-changes Changes and improvements
+
+@subsubsection changelog-latest-changes-trade Trade library
+
+-   For better usability, the @ref magnum-imageconverter "magnum-imageconverter"
+    utility now prints available importer/converter plugins when loading a
+    plugin fails
 
 @section changelog-2019-10 2019.10
 

src/Magnum/Trade/imageconverter.cpp

@@ -156,14 +156,20 @@ key=true.)")
         args.value("plugin-dir").empty() ? std::string{} :
         Utility::Directory::join(args.value("plugin-dir"), Trade::AbstractImporter::pluginSearchPaths()[0])};
     Containers::Pointer<Trade::AbstractImporter> importer = importerManager.loadAndInstantiate(args.value("importer"));
-    if(!importer) return 1;
+    if(!importer) {
+        Debug{} << "Available importer plugins:" << Utility::String::join(importerManager.aliasList(), ", ");
+        return 1;
+    }
 
     /* Load converter plugin */
     PluginManager::Manager<Trade::AbstractImageConverter> converterManager{
         args.value("plugin-dir").empty() ? std::string{} :
         Utility::Directory::join(args.value("plugin-dir"), Trade::AbstractImageConverter::pluginSearchPaths()[0])};
     Containers::Pointer<Trade::AbstractImageConverter> converter = converterManager.loadAndInstantiate(args.value("converter"));
-    if(!converter) return 2;
+    if(!converter) {
+        Debug{} << "Available converter plugins:" << Utility::String::join(converterManager.aliasList(), ", ");
+        return 2;
+    }
 
     /* Set options, if passed */
     setOptions(*importer, args.value("importer-options"));