Update NotificationTechnicalFailureException

- Change the NotificationTechnicalFailureException so that it only inherits from Exception.
- The notify_celery task should create the logging message on failure.
- Fix unit tests
- Remove named parameter when raising exception.

tests/app/celery/test_ftp_update_tasks.py

@@ -173,9 +173,7 @@ def test_update_letter_notifications_to_sent_to_dvla_updates_based_on_notificati
 
 
 def test_update_letter_notifications_to_error_updates_based_on_notification_references(
-    client,
-    sample_letter_template,
-    mocker
+    sample_letter_template
 ):
     first = create_notification(sample_letter_template, reference='first ref')
     second = create_notification(sample_letter_template, reference='second ref')
@@ -184,7 +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.message
+            assert first.reference in e.value
 
     assert first.status == NOTIFICATION_TECHNICAL_FAILURE
     assert first.sent_by is None