exceptions.py: Don't assume message is always present

Change-Id: I71e6fcbfb975eb4771489ad60889a548a1b4d28a
Reviewed-on: http://review.couchbase.org/29773
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>