raise ValueError when user attempts to get status for failed import

Author: Matt Sokoloff

labelbox/schema/annotation_import.py

@@ -124,7 +124,7 @@ def _fetch_remote_ndjson(self, url: str) -> List[Dict[str, Any]]:
             ndjson as a list of dicts.
         """
         if self.state == AnnotationImportState.FAILED:
-            raise Exception("")
+            raise ValueError("Import failed.")
 
         response = requests.get(url)
         response.raise_for_status()