Fix traceback on rare agent error case

If self._deploy_is_done() fails for any reason when called on a agent
heartbeat, 'msg' was not set, creating a traceback when trying to log
it. Let's set a default.

Co-Authored-By: Jim Rollenhagen <jim@jimrollenhagen.com>

Change-Id: I7399821019bab4e3cf08324ee3314c40467e00a4