Better error message in app.window.current() when called from bg page.

chrome/renderer/resources/extensions/app_window_custom_bindings.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.
 
 // Custom binding for the app_window API.
 
 var appWindowNatives = requireNative('app_window_natives');
 var Binding = require('binding').Binding;
 var chrome = requireNative('chrome').GetChrome();
 var Event = require('event_bindings').Event;
@@ skipping 55 matching lines @@
         }
       });
       if (!willCallback) {
         callback(undefined);
       }
     }
   });
 
   apiFunctions.setHandleRequest('current', function() {
     if (!currentAppWindow) {
-      console.error('chrome.app.window.current() is null -- window not ' +
-                    'created with chrome.app.window.create()');
+      console.error('The JavaScript context calling ' +
+                    'chrome.app.window.current() has no associated AppWindow.');
       return null;
     }
     return currentAppWindow;
   });
 
   // This is an internal function, but needs to be bound with setHandleRequest
   // because it is called from a different JS context.
   apiFunctions.setHandleRequest('initializeAppWindow', function(params) {
     var currentWindowInternal =
         Binding.create('app.currentWindowInternal').generate();
@@ skipping 75 matching lines @@
 
 function onAppWindowClosed() {
   if (!currentAppWindow)
     return;
   currentAppWindow.onClosed.dispatch();
 }
 
 exports.binding = appWindow.generate();
 exports.onAppWindowClosed = onAppWindowClosed;
 exports.updateAppWindowProperties = updateAppWindowProperties;