Merge "releasetools: Capture stderr output when calling delta_generator."

  cmd = ['delta_generator',

         '--in_file=' + payload_file,

         '--public_key=' + pubkey]

  proc = common.Run(cmd, stdout=subprocess.PIPE)

  proc = common.Run(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)

  stdoutdata, _ = proc.communicate()

  assert proc.returncode == 0, \

      'Failed to verify payload with delta_generator: %s\n%s' % (package,