[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

Well.. think about when the user phones up and is like 

  "I have an error. It is a red bar at the top of the screen, and it says "Error 500"
 
Then think about how much easier it would be if they said:
 
  "I have an error. It is a red bar at the top of the screen, and it says "Error 500: Cannot find content type foo"

An error message is less cryptic by far than no message at all.