[bug] Unknown notification type breaks API

[nemesifier]

Try the following:

• create a notification with ./manage create_notification
• stop registering the default notification type
• try opening the API

Expected result:

• empty response shows and the notification just created should be deleted automatically

Actual result:

500 API internal server error.

I can replicate this bug in prod.

I am not sure if it's related to #101 or not, my impression is that the work we did on this front is not working as expected and should be fixed.