Log error info when exception occurred.

def process_exception(self, request, exception):
        """Log error info when exception occurred."""
        log_format = self._get_log_format(request)
        if log_format is None:
            return

        params = self._get_parameters_from_request(request, True)
        params['message'] = exception
        params['http_status'] = '-'

        self.OPERATION_LOG.info(log_format, params)