[Admin] Show notification on unhandled AJAX errors

This PR adds an error notification in the UI when 500 or other "unhandled" errors are returned from the server.

__Tasks:__

- [ ] gather feedback for my changes

For me it doesn't make sense to write the symfony exception to the user interface. For the user it is just cryptic, and the developer will have a deeper look at the request or the log anyway. /cc @berndhep