Add a check when people specify wrong change list name to give a meaningful message.

gcl.py

 #!/usr/bin/python
 # Copyright (c) 2006-2009 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.
 
 """\
 Wrapper script around Rietveld's upload.py that simplifies working with groups
 of files.
 """
 
@@ skipping 941 matching lines @@
   change_info = ChangeInfo.Load(changename, GetRepositoryRoot(), False, True)
   silent = FilterFlag(args, "--silent")
 
   # Verify the user is running the change command from a read-write checkout.
   svn_info = SVN.CaptureInfo('.')
   if not svn_info:
     ErrorExit("Current checkout is unversioned.  Please retry with a versioned "
               "directory.")
 
   if len(args) == 2:
+    if not os.path.isfile(args[1]):
+      ErrorExit('The change "%s" doesn\'t exist.' % args[1])
     f = open(args[1], 'rU')
     override_description = f.read()
     f.close()
   else:
     override_description = None
 
   if change_info.issue and not change_info.NeedsUpload():
     try:
       description = GetIssueDescription(change_info.issue)
     except urllib2.HTTPError, err:
@@ skipping 350 matching lines @@
     if command:
       return command(argv[1:])
     # Unknown command, try to pass that to svn
     return CMDpassthru(argv)
   except gclient_utils.Error, e:
     print('Got an exception')
     print(str(e))
 
 if __name__ == "__main__":
   sys.exit(main(sys.argv[1:]))