Conveniently retrieves message contents from a batch file. Can throw a
        :class:`python:RuntimeError` if the message batch filename doesn't
        match this batch filename.

def getMessage(self, message):
        '''
        Conveniently retrieves message contents from a batch file. Can throw a
        :class:`python:RuntimeError` if the message batch filename doesn't
        match this batch filename.
        '''
        if message.filename != self._filename:
            raise RuntimeError(
                'Message filename ({:}) does not match batch filename ({:}).'
                .format(message.filename, self._filename))
        self._file.seek(message.offset)
        self._logger.debug(
            'Retrieved message offset={:}, size={:}'
            .format(message.offset, message.size))
        return self._file.read(message.size)