Add some error messages for setting wallpaper failures on Chrome(c++) side.

chrome/browser/resources/chromeos/wallpaper_manager/js/butter_bar.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * The minimum about of time to display the butter bar for, in ms.
  * Justification is 1000ms for minimum display time plus 300ms for transition
  * duration.
  */
 var MINIMUM_BUTTER_DISPLAY_TIME_MS = 1300;
@@ skipping 240 matching lines @@
    this.show(loadTimeData.getString('downloadCanceled'), {timeout: 1000});
    this.xhr_ = null;
 };
 
 /**
  * Hides butter bar when download complete.
  * @private
  * @param {Event} e A load ProgressEvent from XMLHttpRequest.
  */
 ButterBar.prototype.onDownloadComplete_ = function(e) {
-   this.hide_();
    this.xhr_ = null;
 };
 
 /**
  * Shows error message when receiving an error event.
  * @private
  * @param {Event} e An error ProgressEvent from XMLHttpRequest.
  */
 ButterBar.prototype.onDownloadError_ = function(e) {
   this.showError_(loadTimeData.getString('downloadFailed'));
   this.xhr_ = null;
 };
 
 /**
  * Calculates downloading percentage and shows downloading progress.
  * @private
  * @param {Event} e A progress ProgressEvent from XMLHttpRequest.
  */
 ButterBar.prototype.onDownloadProgress_ = function(e) {
   if (e.lengthComputable) {
     this.percentComplete_ = e.loaded / e.total;
   }
   this.showProgress_();
 };