def errReceived(self, data):
        """
        Called when STDERR from the process has output; if we have not yet
        successfully deserialized a JSON message, logs STDERR at debug-level;
        otherwise discards it.

        :param data: STDERR from the process
        :type data: str
        """
        if not self.have_data:
            logger.debug('Process STDERR: %s', data)