function handleError(gltfLoader, error) {
  gltfLoader.unload();
  const errorMessage = "Failed to load glTF";
  throw gltfLoader.getError(errorMessage, error);
}
Loads the resource.
@returns {Promise.<GltfLoader>} A promise which resolves to the loader when the resource loading is completed.
@exception {RuntimeError} Unsupported glTF version
@exception {RuntimeError} Unsupported glTF Extension
@private