Fix assertions when we catch an error in the tests

Code that is within a `with Python.raises(...)` context manager but
comes after the line that raises the exception doesn't get evaluated.
We had some assertions that we never being tested because of this, so
this ensures that they will always get run and fixes them where
necessary.

tests/app/celery/test_ftp_update_tasks.py

@@ -182,7 +182,7 @@ def test_update_letter_notifications_to_error_updates_based_on_notification_refe
     with freeze_time(dt):
         with pytest.raises(NotificationTechnicalFailureException) as e:
             update_letter_notifications_to_error([first.reference])
-            assert first.reference in e.value
+    assert first.reference in str(e.value)
 
     assert first.status == NOTIFICATION_TECHNICAL_FAILURE
     assert first.sent_by is None