Redirect the stderr to a log file.

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