Add more error message if the script failed to pull result file from device.

  result_file = "/sdcard/load_test_result.txt"

  shell_cmd_str = adb_cmd + " pull " + result_file + " " + results_dir

  adb_output = subprocess.Popen(shell_cmd_str, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE).communicate()[0]

  logging.info(adb_output)

  (adb_output, err) = subprocess.Popen(

      shell_cmd_str, shell=True,

      stdout=subprocess.PIPE, stderr=subprocess.PIPE).communicate()

  if not os.path.isfile(os.path.join(results_dir, "load_test_result.txt")):

    logging.error("Failed to pull result file.")

    logging.error("adb stdout:")

    logging.error(adb_output)

    logging.error("adb stderr:")

    logging.error(err)

  logging.info("Results are stored under: " + results_dir + "/load_test_result.txt\n")

if '__main__' == __name__: