Make it clear that syncFS getrequestFileSystem can fail.

chrome/common/extensions/api/sync_file_system.idl

 // 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.
 
 // Use the <code>chrome.syncFileSystem</code> API to save and synchronize data
 // on Google Drive. This API is NOT for accessing arbitrary user docs stored in
 // Google Drive. It provides app-specific syncable storage for offline and
 // caching usage so that the same data can be available across different
 // clients. Read <a href="app_storage.html">Manage Data</a> for more on using
 // this API.
@@ skipping 128 matching lines @@
     // in the same way as the Temporary and Persistant file systems (see
     // <a href="http://www.w3.org/TR/file-system-api/">http://www.w3.org/TR/file-system-api/</a>),
     // except that the filesystem object returned for Sync FileSystem does
     // <b>NOT</b> support directory operations (yet). You can get a list
     // of file entries by reading the root directory (by
     // <a href="http://www.w3.org/TR/file-system-api/#widl-DirectoryEntry-createReader-DirectoryReader">creating a new DirectoryReader</a>),
     // but cannot create a new directory in it.
     //
     // Calling this multiple times from
     // the same app will return the same handle to the same file system.
+    //
+    // Note this call can fail. For example, if the user is not signed in to
+    // Chrome or if there is no network operation. To handle these errors it is
+    // important chrome.runtime.lastError is checked in the callback.
     static void requestFileSystem(GetFileSystemCallback callback);
 
     // Sets the default conflict resolution policy
     // for the <code>'syncable'</code> file storage for the app.
     // By default it is set to <code>'last_write_win'</code>.
     // When conflict resolution policy is set to <code>'last_write_win'</code>
     // conflicts for existing files are automatically resolved next time
     // the file is updated.
     // |callback| can be optionally given to know if the request has
     // succeeded or not.
@@ skipping 31 matching lines @@
     // Fired when an error or other status change has happened in the
     // sync backend (for example, when the sync is temporarily disabled due to
     // network or authentication error).
     static void onServiceStatusChanged(ServiceInfo detail);
 
     // Fired when a file has been updated by the background sync service.
     static void onFileStatusChanged(FileInfo detail);
   };
 
 };