handle doc dl connection errors correctly

previously we'd see an error message in the logs:
`AttributeError: 'NoneType' object has no attribute 'status_code'`
because we were assuming the requests exception would always have a
response - it won't have a response if it wasn't able to create a
connection at all.

app/clients/document_download.py

@@ -42,7 +42,9 @@ class DocumentDownloadClient:
             # if doc dl responds with a non-400, (eg 403) it's referring to credentials that the API and Doc DL use.
             # we don't want to tell users about that, so anything that isn't a 400 (virus scan failed or file type
             # unrecognised) should be raised as a 500 internal server error here.
-            if e.response.status_code == 400:
+            if e.response is None:
+                raise Exception(f'Unhandled document download error: {repr(e)}')
+            elif e.response.status_code == 400:
                 error = DocumentDownloadError.from_exception(e)
                 current_app.logger.info(
                     'Document download request failed with error: {}'.format(error.message)