[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

Not if its not your server.

But I really don't think you can confuse the user more by not showing them the actual error. In either case the system has failed and it shouldn't do that. The least it can do is give you a hint which you can use to describe your issue.