Modify DeviceHandler to handle deviceFail event and add a message.

chrome/test/data/file_manager/unit_tests/device_handler_unittest.js

 // Copyright 2014 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.
 'use strict';
 
 /**
  * Test target.
  * @type {DeviceHandler}
  */
 var handler;
@@ skipping 38 matching lines @@
         }
       },
       onMountCompleted: {
         addListener: function(listener) {
           this.dispatch = listener;
         }
       }
     },
     notifications: {
       create: function(id, params, callback) {
-        assertFalse(!!this.items[id]);
         this.items[id] = params;
         callback();
       },
       clear: function(id, callback) { delete this.items[id]; callback(); },
       items: {},
       onButtonClicked: {
         addListener: function(listener) {
           this.dispatch = listener;
         }
       }
@@ skipping 348 matching lines @@
 function testDeviceHardUnplugged() {
   chrome.fileBrowserPrivate.onDeviceChanged.dispatch({
     type: 'hard_unplugged',
     devicePath: '/device/path'
   });
   assertEquals(1, Object.keys(chrome.notifications.items).length);
   assertEquals('DEVICE_HARD_UNPLUGGED_MESSAGE',
                chrome.notifications.items[
                    'hardUnplugged:/device/path'].message);
 }