Admin doesn't handle new on_delete exceptions gracefully
|Reported by:||Jani Tiainen||Owned by:||Carl Meyer|
|Has patch:||no||Needs documentation:||no|
|Needs tests:||no||Patch needs improvement:||no|
Changes made in #7539 causes now admin to throw HTTP 500 when integrity exception happens leaving end user pretty much clueless that something "normal" has happened.