Add proper handling for foreign key exceptions

[Taucher2003]

We have operations where we delete entities and don't handle errors on foreign keys resulting in an error response to the client.

Either we should add checks that the entity we want to delete is not being referenced or we should rescue the foreign key exception and return a generic error for these cases

[Taucher2003]

Deleting entities that are still referenced and therefore cause foreign key exceptions is always a server side bug. Those entities should not get deleted in the first place.

Handling those with an error response like we do for actual client errors isn't appropriate here because it would "put the blame" on the client. These cases should continue to fail with 500 Internal server error because that is what these things actually are.