fix stderr format

tools/testing/frogpad/frogpad.py

 #!/usr/bin/env python
 
 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 #
 
 """
 Frogpad is used to compile .dart files to javascript.
 
@@ skipping 327 matching lines @@
   """
 
   command = format_command(args)
   logging.debug("RUNNING " + command)
   child = subprocess.Popen(args,
       stdout=subprocess.PIPE,
       stderr=subprocess.PIPE,
       close_fds=True)
   (stdout, stderr) = child.communicate()
   for line in stderr.splitlines():
-    logging.info(logging.INFO, '%s: %s', args[0], line)
+    logging.info(logging.INFO, '%s: %s' % (args[0], line))
   exit_code = child.wait()
   if exit_code:
     msg = "FAILURE (exit_code=%d): '%s'" % (exit_code, command)
     logging.error(msg)
     raise CommandFailedException(msg)
   logging.debug("SUCCEEDED (%d bytes)" % len(stdout))
   return stdout
 
 
 def main(argv):
   Pad(argv)
 
 if __name__ == "__main__":
   sys.exit(main(sys.argv))