Sanity check the message.

        If we have PLAIN and HTML versions, send a multipart alternative
        MIME message, else send whichever we do have.

        If we have neither, raise NoContentError

        Arguments:
        - `sender`: str
        - `to`: list
        - `subject`: str
        - `plain`: str
        - `html`: str

        Return: None
        Exceptions: NoContentError

def sanity_check(self, sender, to, subject, plain=None, html=None, cc=None, bcc=None):
        """
        Sanity check the message.

        If we have PLAIN and HTML versions, send a multipart alternative
        MIME message, else send whichever we do have.

        If we have neither, raise NoContentError

        Arguments:
        - `sender`: str
        - `to`: list
        - `subject`: str
        - `plain`: str
        - `html`: str

        Return: None
        Exceptions: NoContentError
        """
        if not plain and not html:
            raise NoContentError()