python: test trade.AbstractImporter.mesh() failure.

src/python/magnum/test/mesh.gltf

@@ -30,6 +30,14 @@
           }
         }
       ]
+    },
+    {
+      "name": "A broken mesh",
+      "primitives": [
+        {
+          "mode": 666
+        }
+      ]
     }
   ],
   "accessors": [

src/python/magnum/test/test_trade.py

@@ -706,7 +706,7 @@ class Importer(unittest.TestCase):
         self.assertIsNone(importer.mesh_attribute_for_name("_CUSTOM_ATTRIBUTE"))
 
         importer.open_file(os.path.join(os.path.dirname(__file__), 'mesh.gltf'))
-        self.assertEqual(importer.mesh_count, 2)
+        self.assertEqual(importer.mesh_count, 3)
         self.assertEqual(importer.mesh_level_count(0), 1)
         self.assertEqual(importer.mesh_name(0), 'Indexed mesh')
         self.assertEqual(importer.mesh_for_name('Indexed mesh'), 0)
@@ -753,6 +753,15 @@ class Importer(unittest.TestCase):
         with self.assertRaises(IndexError):
             importer.mesh('Non-indexed mesh', 1)
 
+    def test_mesh_failed(self):
+        importer = trade.ImporterManager().load_and_instantiate('GltfImporter')
+        importer.open_file(os.path.join(os.path.dirname(__file__), 'mesh.gltf'))
+
+        with self.assertRaisesRegex(RuntimeError, "import failed"):
+            importer.mesh(2)
+        with self.assertRaisesRegex(RuntimeError, "import failed"):
+            importer.mesh('A broken mesh')
+
     def test_image2d(self):
         manager = trade.ImporterManager()
         manager_refcount = sys.getrefcount(manager)