Don't return 409 status code when run_txn fails.

Change-Id: Idddf5cf14477b5c85357c7a87f5259931185a925
Reviewed-on: http://review.couchbase.org/34356
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>