Notifications are silently ignored if not also defined in app-notifications-service

[dorthrithil]

This:

toastNotification(
  NotificationIdentifier.INSTANCE_SETTINGS_ERROR_MIGRATION_IMPOSSIBLE,
)

will do nothing, if the key is not defined in the notifications service. It should at least throw an error.

But a better way is to refactor this construct, so we don't have the enum and the map in some de-coupled files.