XML submodel: log the model name in the error.

Try to improve diagnosis of problems where overlay XML Nodes are used,
and the model path might be empty, as happened with the 737-800

simgear/scene/model/SGReaderWriterXML.cxx

@@ -673,7 +673,7 @@ sgLoad3DModel_internal(const SGPath& path,
         if (submodelPath.isNull()) {
           SG_LOG(SG_IO, SG_DEV_ALERT, "Failed to load file: \"" << subPathStr << "\"");
           simgear::reportFailure(simgear::LoadFailure::NotFound, simgear::ErrorCode::XMLModelLoad,
-                                 "Couldn't find file for submodel:" + subPathStr,
+                                 "Couldn't find file for submodel '" + sub_props->getStringValue("name") + "': " + subPathStr,
                                  SGPath::fromUtf8(subPathStr));
           continue;
         }