Materials: Catch correct error

When the UUID is set to a non-existent material, it was Catching
Materials::ModelNotFound instead of Materials::MaterialNotFound

src/Mod/Material/Gui/MaterialTreeWidget.cpp

@@ -273,7 +273,7 @@ void MaterialTreeWidget::updateMaterial(const QString& uuid)
     try {
         material = std::make_shared<Materials::Material>(*getMaterialManager().getMaterial(uuid));
     }
-    catch (Materials::ModelNotFound const&) {
+    catch (Materials::MaterialNotFound const&) {
         Base::Console().Log("*** Unable to load material '%s'\n", uuid.toStdString().c_str());
     }