Merge branch 'global-ajax-error-handler' into 'master'

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

CHANGELOG

@@ -245,6 +245,7 @@ v 8.9.0 (unreleased)
   - Filter parameters for request_uri value on instrumented transactions.
   - Remove duplicated keys add UNIQUE index to keys fingerprint column
   - ExtractsPath get ref_names from repository cache, if not there access git.
+  - Show a flash warning about the error detail of XHR requests which failed with status code 404 and 500
   - Cache user todo counts from TodoService
   - Ensure Todos counters doesn't count Todos for projects pending delete
   - Add left/right arrows horizontal navigation

app/assets/javascripts/application.js.coffee

@@ -185,6 +185,15 @@ $ ->
       else
         buttons.enable()
 
+  $(document).ajaxError (e, xhrObj, xhrSetting, xhrErrorText) ->
+
+    if xhrObj.status is 401
+      new Flash 'You need to be logged in.', 'alert'
+
+    else if xhrObj.status in [ 404, 500 ]
+      new Flash 'Something went wrong on our end.', 'alert'
+
+
   # Show/Hide the profile menu when hovering the account box
   $('.account-box').hover -> $(@).toggleClass('hover')