PYCBC-241: Expose semantic error codes

Change-Id: Ic412362364ab4cebde5aef3d02a075b03ef9e585
Reviewed-on: http://review.couchbase.org/35689
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>