User data not deleted if User::delete failed

[susnux]

Current flow is:

1. Emit BeforeUserDeletedEvent
2. Delete user from backend
3. Delete core user data
4. Emit UserDeletedEvent
5. (listern on that event: Remove user files)

If anywhere between 2 and 5 and exception occurs, e.g. database connection is lost, the state is undefined.
This means the user is already removed from backend, so we have no associated user anymore, but some data might still be available.
It is not possible to recover from this state because:

1. The user is gone (we could check if there are remnants by checking user preferences for userIds not available on any registered backend)
2. Our API (especially the events to let apps delete their user data) require real users not only user ids

While this case is probably logged (unexpected exception), removing all user data is hard for admins as it is unclear which tables and files to remove.