Global Ajax error handler

## What does this MR do?
Introduces a new global Ajax error handler to show a flash warning about the error detail of XHR requests which failed with status code 404 and 500

## Are there points in the code the reviewer needs to double check?
Double check the wording.

## Why was this MR needed?
A global solution for the problem in the #17880

## What are the relevant issue numbers?
Fixes #17880 

## Screenshots (if relevant)

For requests failed with 404 - 500 

![Screen_Shot_2016-06-21_at_00.07.26](/uploads/04bb3a5a958c3d21aef4fd12bd3d105a/Screen_Shot_2016-06-21_at_00.07.26.png)

For requests failed with 401

![Screen_Shot_2016-06-21_at_00.08.30](/uploads/25cc885ba388f4c6f4c555fa338e1a63/Screen_Shot_2016-06-21_at_00.08.30.png)

See merge request !4797