docs: 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)