Add HTTPError to exceptions documentation

Change-Id: Ieb47ccac8a1b5d4572fa5e5a9bb1271dc5440a10
Reviewed-on: http://review.couchbase.org/26770
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>