added value error to docstring

Author: andrewelamb

synapseclient/models/curation.py

@@ -641,7 +641,8 @@ async def delete_async(
 
         Raises:
             ValueError: If the CurationTask object does not have a task_id.
-
+            ValueError: If delete_source is True but the task properties are not properly set
+              to identify the source to delete.
 
         Example: Delete a curation task asynchronously