PYCBC-232: Wrap transcoder errors in CouchbaseError

Change-Id: Ib7a26f15855e38a7ee8c9727a341761919b65ef7
Reviewed-on: http://review.couchbase.org/33908
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>