Вывод сообщения при отсутствии аргументов

$ ./phone_photos_to_nas.py 
Traceback (most recent call last):
  File "./phone_photos_to_nas.py", line 76, in <module>
    main(args.inputdir, args.outputdir)
  File "./phone_photos_to_nas.py", line 15, in main
    check_dir(inputdir)
  File "./phone_photos_to_nas.py", line 57, in check_dir
    if not os.path.isdir(path_to_dir):
  File "/usr/lib/python2.7/genericpath.py", line 41, in isdir
    st = os.stat(s)
TypeError: coercing to Unicode: need string or buffer, NoneType found