Use exit code 21 in GitHub action if working-directory is missing

[akaihola]

For error handling, it would be useful to have a unique exit code from the Darker GitHub action if the directory specified as working-directory doesn't exist.

Here's the current behavior:

Traceback (most recent call last):
  File "/home/runner/work/darker/darker/action/main.py", line 59, in <module>
    proc = run(  # nosec
  File "/usr/lib/python3.10/subprocess.py", line 503, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/usr/lib/python3.10/subprocess.py", line 971, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib/python3.10/subprocess.py", line 1863, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: '/home/runner/work/_temp/non-existent-dir'
##[error]Process completed with exit code 1.

Darker itself uses an exit code of 2 if a file doesn't exist. For example 21 as a variation of that code could be used for the action.