Revert "Revert "Improve ajax errors [MAILPOET-929]""

This reverts commit 81c3e2facf.

assets/js/src/ajax.js

@@ -1,5 +1,19 @@
+function requestFailed(errorMessage, xhr) {
+  if (xhr.responseJSON) {
+    return xhr.responseJSON;
+  }
+  var message = errorMessage.replace("%d", xhr.status);
+  return {
+    errors: [
+      {
+        message: message
+      }
+    ]
+  }
+}
+
 define('ajax', ['mailpoet', 'jquery', 'underscore'], function(MailPoet, jQuery, _) {
-  'use strict';
+
   MailPoet.Ajax = {
       version: 0.5,
       options: {},
@@ -44,7 +58,6 @@ define('ajax', ['mailpoet', 'jquery', 'underscore'], function(MailPoet, jQuery,
 
         // set request params
         var params = this.getParams();
-        var deferred = jQuery.Deferred();
 
         // remove null values from the data object
         if (_.isObject(params.data)) {
@@ -54,16 +67,14 @@ define('ajax', ['mailpoet', 'jquery', 'underscore'], function(MailPoet, jQuery,
         }
 
         // ajax request
-        deferred = jQuery.post(
+        var deferred = jQuery.post(
           this.options.url,
           params,
           null,
           'json'
         ).then(function(data) {
           return data;
-        }, function(xhr) {
+        }, _.partial(requestFailed, MailPoet.I18n.t('ajaxFailedErrorMessage')));
-          return xhr.responseJSON;
-        });
 
         // clear options
         this.options = {};

views/layout.html

@@ -55,6 +55,9 @@ jQuery('.toplevel_page_mailpoet-newsletters.menu-top-last')
   'mailpoet.js'
 )%>
 
+<%= localize({
+  'ajaxFailedErrorMessage': __('An error has happened while performing a request, the server has responded with response code %d'),
+}) %>
 <% block translations %><% endblock %>
 
 <% block after_translations %><% endblock %>