minor chg, debug dataset viewer

glenda.py

@@ -224,6 +224,7 @@ class GLENDA(datasets.GeneratorBasedBuilder):
             except AttributeError:
                 if match is None:
                     print("Could not get metadata for: ", image_filename_with_parent_folder)
+                    raise ValueError(image_filename_with_parent_folder)
                 metadata.update({
                     "video_id": None,
                     "frame_id": None,