Make setup_links.py not fail if Chromium checkout is missing.

setup_links.py

 #!/usr/bin/env python
 # Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 """Setup links to a Chromium checkout for WebRTC.
@@ skipping 489 matching lines @@
     def is_admin():
       try:
         return os.getuid() == 0
       except AttributeError:
         return ctypes.windll.shell32.IsUserAnAdmin() != 0
     if is_admin():
       logging.warning('WARNING: On Windows, you no longer need run as '
                       'administrator. Please run with user account privileges.')
 
   if not os.path.exists(CHROMIUM_CHECKOUT):
-    logging.error('Cannot find a Chromium checkout at %s. Did you run "gclient '
-                  'sync" before running this script?', CHROMIUM_CHECKOUT)
-    return 2
+    logging.warning('Cannot find a Chromium checkout at %s. Did you run '
+                    '"gclient sync" before running this script?',
+                    CHROMIUM_CHECKOUT)
+    return 0
 
   links_database = _initialize_database(LINKS_DB)
   try:
     symlink_creator = WebRTCLinkSetup(links_database, options.force,
                                       options.dry_run, options.prompt)
     symlink_creator.CleanupLinks()
     if not options.clean_only:
       symlink_creator.CreateLinks(on_bot)
   except LinkError as e:
     print >> sys.stderr, e.message
     return 3
   finally:
     links_database.close()
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main())