Make gn.py support root directories other than 'src'.

gn.py

 #!/usr/bin/env python
 # Copyright 2013 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.
 
 """This script is a wrapper around the GN binary that is pulled from Google
 Cloud Storage when you sync Chrome. The binaries go into platform-specific
 subdirectories in the source tree.
 
 This script makes there be one place for forwarding to the correct platform's
 binary. It will also automatically try to find the gn binary when run inside
 the chrome source tree, so users can just type "gn" on the command line
 (normally depot_tools is on the path)."""
 
 import gclient_utils
 import os
 import subprocess
 import sys
 
 
 def main(args):
   bin_path = gclient_utils.GetBuildtoolsPlatformBinaryPath()
   if not bin_path:
     print >> sys.stderr, ('gn.py: Could not find checkout in any parent of '
                           'the current path.\nThis must be run inside a '
                           'checkout.')
-    sys.exit(1)
+    return 1
   gn_path = os.path.join(bin_path, 'gn' + gclient_utils.GetExeSuffix())
-  return subprocess.call([gn_path] + sys.argv[1:])
+  if not os.path.exists(gn_path):
+    print >> sys.stderr, 'gn.py: Could not find gn executable at: %s' % gn_path
+    return 2
+  else:
+    return subprocess.call([gn_path] + sys.argv[1:])
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))