<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def fetch(self):
 """Fetch data corresponding to this draft and store it as ``self.data``."""

        if self.message_id is None:
            raise Exception(".message_id not set.")
        response = self.session.request("find:Message.content", [ self.message_id ])
        if response == None:
            raise Exception("Message not found.")
        self.data = response
        return self