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

chrome/renderer/resources/extensions/app_window_custom_bindings.js

Index: chrome/renderer/resources/extensions/app_window_custom_bindings.js
diff --git a/chrome/renderer/resources/extensions/app_window_custom_bindings.js b/chrome/renderer/resources/extensions/app_window_custom_bindings.js
index f6171e192ce133efe9e4dec631ba26779377e323..679aa0075670e4119220115e2af223bad5f4cf28 100644
--- a/chrome/renderer/resources/extensions/app_window_custom_bindings.js
+++ b/chrome/renderer/resources/extensions/app_window_custom_bindings.js
@@ -73,8 +73,8 @@ appWindow.registerCustomHook(function(bindingsAPI) {
 
   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;