get_message should exit gracefully with empty cache

[slz]

Currently, on a new Zesty debathena-login install, get_message prints a scary trackback on user login. This happens when it could not obtain new message, and the message cache is empty, causing parse() to fail. We should silently ignore this error.

[slz]

Traceback (most recent call last):
 File "./get_message", line 114, in <module>
   main()
 File "./get_message", line 104, in main
   version, timestamp, content = get_message()
 File "./get_message", line 88, in get_message
   return parse(msg)
 File "./get_message", line 34, in parse
   raise Exception('Unable to parse the specified message')
Exception: Unable to parse the specified message