def _read_stderr(self):
        '''Continuously read stderr for error messages.'''
        try:
            while self._process.returncode is None:
                line = yield from self._process.stderr.readline()

                if not line:
                    break

                if self._stderr_callback:
                    yield from self._stderr_callback(line)

        except Exception:
            _logger.exception('Unhandled read stderr exception.')
            raise
Continuously read stderr for error messages.