Ensure that BotServerError.message is properly returned.

BotoServerError inherits from StandardError which defines the message attribute (to None if it isn't defined as the lone parameter to the constructor). This causes the __getattr__ lookup to find the superclass version of "message" which is None resulting in unexpected behavior when getting the message from a server failure.

Instead of __getattr__ for the 'message' attribute, set it after parsing the XML in the constructor.