docs: def stderr_cm(self):
          """ Redirect the stderr to a log file. """
          with open(self._get_stderr_log(), 'a+') as fh:
              msg = '### {} ###\n'.format(self._datetime)
              fh.write(msg)
              fh.flush()
  
              try:
                  yield fh
              except Exception as e:
                  self._has_error = True
                  fh.write(e.message)
                  fh.flush()
                  raise