python: distinguish OOB ID and OOB level in trade.AbstractImporter.

So when it fails, it's possible to know on which.

src/python/magnum/test/test_trade.py

@@ -657,7 +657,7 @@ class Importer(unittest.TestCase):
             importer.mesh_level_count(0)
         with self.assertRaises(IndexError):
             importer.mesh_name(0)
-        with self.assertRaises(IndexError):
+        with self.assertRaisesRegex(IndexError, "ID out of bounds"):
             importer.mesh(0)
 
         with self.assertRaises(IndexError):
@@ -674,13 +674,13 @@ class Importer(unittest.TestCase):
         with self.assertRaises(IndexError):
             importer.image3d_name(0)
 
-        with self.assertRaises(IndexError):
+        with self.assertRaisesRegex(IndexError, "ID out of bounds"):
             importer.image1d(0)
-        with self.assertRaises(IndexError):
+        with self.assertRaisesRegex(IndexError, "level out of bounds"):
             importer.image2d(0, 1)
-        with self.assertRaises(IndexError):
+        with self.assertRaisesRegex(IndexError, "ID out of bounds"):
             importer.image2d(1)
-        with self.assertRaises(IndexError):
+        with self.assertRaisesRegex(IndexError, "ID out of bounds"):
             importer.image3d(0)
 
     def test_open_failed(self):
@@ -750,7 +750,7 @@ class Importer(unittest.TestCase):
         importer = trade.ImporterManager().load_and_instantiate('GltfImporter')
         importer.open_file(os.path.join(os.path.dirname(__file__), 'mesh.gltf'))
 
-        with self.assertRaises(IndexError):
+        with self.assertRaisesRegex(IndexError, "level out of bounds"):
             importer.mesh('Non-indexed mesh', 1)
 
     def test_mesh_failed(self):
@@ -790,7 +790,7 @@ class Importer(unittest.TestCase):
         importer = trade.ImporterManager().load_and_instantiate('StbImageImporter')
         importer.open_file(os.path.join(os.path.dirname(__file__), 'rgb.png'))
 
-        with self.assertRaises(IndexError):
+        with self.assertRaisesRegex(IndexError, "level out of bounds"):
             importer.image2d(0, 1)
 
     def test_image2d_by_name(self):

src/python/magnum/trade.cpp

@@ -329,12 +329,12 @@ template<class R, Containers::Optional<R>(Trade::AbstractImporter::*f)(UnsignedI
     }
 
     if(id >= (self.*bounds)()) {
-        PyErr_SetNone(PyExc_IndexError);
+        PyErr_SetString(PyExc_IndexError, "ID out of bounds");
         throw py::error_already_set{};
     }
 
     if(level >= (self.*levelBounds)(id)) {
-        PyErr_SetNone(PyExc_IndexError);
+        PyErr_SetString(PyExc_IndexError, "level out of bounds");
         throw py::error_already_set{};
     }
 
@@ -362,7 +362,7 @@ template<class R, Containers::Optional<R>(Trade::AbstractImporter::*f)(UnsignedI
     }
 
     if(level >= (self.*levelBounds)(id)) {
-        PyErr_SetNone(PyExc_IndexError);
+        PyErr_SetString(PyExc_IndexError, "level out of bounds");
         throw py::error_already_set{};
     }