AssertionError when retrying a DataImport in a Stopping state

Sentry Issue: ARKINDEX-BACKEND-Q0

While the RetryDataImport forbids retrying a process if its workflow is Unscheduled, Pending or Running, it does not prevent a process from being retried while its workflow is Stopping. Retrying is only allowed when a workflow is in a Stopped, Completed, Failed or Error state. Retrying when a workflow is Stopping causes an AssertionError on Ponos' side.

AssertionError: 
(6 additional frame(s) were not displayed)
...
  File "rest_framework/views.py", line 480, in raise_uncaught_exception
    raise exc
  File "rest_framework/views.py", line 506, in dispatch
    response = handler(request, *args, **kwargs)
  File "arkindex/dataimport/api.py", line 313, in post
  File "arkindex/dataimport/models.py", line 370, in retry
  File "ponos/models.py", line 535, in retry