Build a message.

        :raises InvalidPayloadError:

def build_message(self):
        '''Build a message.

        :raises InvalidPayloadError:
        '''
        if self._text is None:
            raise InvalidPayloadError('text is required')

        message = {
            'text': self._text,
            'markdown': self._markdown
        }

        if self._channel != Incoming.DEFAULT_CHANNEL:
            message['channel'] = self._channel

        if self._attachments:
            message['attachments'] = []
            for attachment in self._attachments:
                if 'title' not in attachment and 'text' not in attachment:
                    raise InvalidPayloadError('title or text is required')
                message['attachments'].append(attachment)

        return message