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

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

Index: chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_manager.js
diff --git a/chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_manager.js b/chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_manager.js
index e432e355f75d3e6883e12db0b1f9127b03f41a27..eb71e4de0abc4ed2210551ccd181d36ccbc58209 100644
--- a/chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_manager.js
+++ b/chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_manager.js
@@ -193,7 +193,8 @@ function WallpaperManager(dialogDom) {
           var image = self.wallpaperRequest_.response;
           chrome.wallpaperPrivate.setWallpaper(image,
                                                selectedItem.layout,
-                                               wallpaperURL);
+                                               wallpaperURL,
+                                               self.onFinished_.bind(self));
           self.currentWallpaper_ = wallpaperURL;
         } else {
           self.butterBar_.showError_(str('downloadFailed'));
@@ -315,11 +316,22 @@ function WallpaperManager(dialogDom) {
     var layout =
         setWallpaperLayout.options[setWallpaperLayout.selectedIndex].value;
     chrome.wallpaperPrivate.setCustomWallpaper(customWallpaper,
-                                               layout);
+                                               layout,
+                                               this.onFinished_.bind(this));
     this.currentWallpaper_ = 'CUSTOM';
   };
 
   /**
+   * Sets wallpaper finished. Displays error message in butter bar if any.
+   */
+  WallpaperManager.prototype.onFinished_ = function() {
+    if (chrome.runtime.lastError != undefined)
+      this.butterBar_.showError_(chrome.runtime.lastError.message);
+    else
+      this.butterBar_.hide_();
+  };
+
+  /**
    * Handles the layout setting change of custom wallpaper.
    */
   WallpaperManager.prototype.onWallpaperLayoutChanged_ = function() {