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

chrome/common/extensions/api/wallpaper_private.json

 // 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.
 
 [
   {
     "namespace":"wallpaperPrivate",
     "nodoc": "true",
     "functions": [
       {
@@ skipping 26 matching lines @@
             "name": "wallpaper"
           },
           {
             "type": "string",
             "name": "layout",
             "enum": [ "STRETCH", "CENTER", "CENTER_CROPPED"]
           },
           {
             "type": "string",
             "name": "url"
+          },
+          {
+            "type": "function",
+            "name": "callback",
+            "parameters": [
+              {
+                "name": "error",
+                "type": "object",
+                "additionalProperties": {"type": "string"}
+              }
+            ]
           }
         ]
       },
       {
         "name": "setCustomWallpaper",
         "type": "function",
         "description": "Sets wallpaper to the image from local file with specified layout",
         "nodoc": "true",
         "parameters": [
           {
             "type": "binary",
             "name": "wallpaper"
           },
           {
             "type": "string",
             "name": "layout",
             "enum": [ "STRETCH", "CENTER", "CENTER_CROPPED"]
+          },
+          {
+            "type": "function",
+            "name": "callback",
+            "parameters": [
+              {
+                "name": "error",
+                "type": "object",
+                "additionalProperties": {"type": "string"}
+              }
+            ]
           }
         ]
       }
     ]
   }
 ]