Fix examples for File System Provider API.

chrome/common/extensions/docs/examples/api/fileSystemProvider/archive/background.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';
 
 // Metadata is stored in files as serialized to JSON maps. See contents of
 // example1.fake and example2.fake.
 
 // Multiple volumes can be opened at the same time. The key is the
@@ skipping 23 matching lines @@
 
 function onUnmountRequested(options, onSuccess, onError) {
   if (Object.keys(volumes[options.fileSystemId].openedFiles).length != 0) {
     onError('IN_USE');
     return;
   }
 
   chrome.fileSystemProvider.unmount(
       {fileSystemId: options.fileSystemId},
       function() {
+        if (chrome.runtime.lastError) {
+          onError(chrome.runtime.lastError.message);
+          return;
+        }
         delete volumes[options.fileSystemId];
         saveState(); // Remove volume from local storage state.
         onSuccess();
-      },
-      function() {
-        onError('FAILED');
       });
 };
 
 function onGetMetadataRequested(options, onSuccess, onError) {
   restoreState(options.fileSystemId, function () {
     var entryMetadata =
         volumes[options.fileSystemId].metadata[options.entryPath];
     if (!entryMetadata)
       error('NOT_FOUND');
     else
@@ skipping 135 matching lines @@
   event.items.forEach(function(item) {
     readMetadataFromFile(item.entry,
         function(metadata) {
           // Mount the volume and save its information in local storage
           // in order to be able to recover the metadata in case of
           // restarts, system crashes, etc.
           chrome.fileSystem.getDisplayPath(item.entry, function(displayPath) {
             volumes[displayPath] = new Volume(item.entry, metadata);
             chrome.fileSystemProvider.mount(
                 {fileSystemId: displayPath, displayName: item.entry.name},
-                function() { saveState(); },
-                function() { console.error('Failed to mount.'); });
+                function() {
+                  if (chrome.runtime.lastError) {
+                    console.error('Failed to mount because of: ' +
+                        chrome.runtime.lastError.message);
+                    return;
+                  };
+                  saveState();
+                });
           });
         },
         function(error) {
           console.error(error);
         });
   });
 });
 
 // Event called on a profile startup.
 chrome.runtime.onStartup.addListener(function () {
@@ skipping 21 matching lines @@
 chrome.fileSystemProvider.onGetMetadataRequested.addListener(
     onGetMetadataRequested);
 chrome.fileSystemProvider.onReadDirectoryRequested.addListener(
     onReadDirectoryRequested);
 chrome.fileSystemProvider.onOpenFileRequested.addListener(
     onOpenFileRequested);
 chrome.fileSystemProvider.onCloseFileRequested.addListener(
     onCloseFileRequested);
 chrome.fileSystemProvider.onReadFileRequested.addListener(
     onReadFileRequested);