Improve ajax errors on form submission

[MAILPOET-929]

lib/Config/Widget.php

@@ -112,6 +112,13 @@ class Widget {
       'ajax_url' => admin_url('admin-ajax.php'),
       'is_rtl' => (function_exists('is_rtl') ? (bool)is_rtl() : false)
     ));
+
+    $ajaxFailedErrorMessage = __('An error has happened while performing a request, please try again later.');
+    wp_add_inline_script(
+      'mailpoet_public',
+      sprintf('MailPoet.I18n.add("ajaxFailedErrorMessage", "%s")', $ajaxFailedErrorMessage),
+      'after'
+    );
   }
 
   function setupAdminWidgetPageDependencies() {

webpack.config.js

@@ -248,6 +248,7 @@ var publicConfig = {
   entry: {
     public: [
       'mailpoet',
+      'i18n',
       'ajax',
       'iframe',
       'jquery.serialize_object',