txcouchbase test_ops: Check res_fail rather than res

This somehow accidentally worked in previous runs, because often
res_fail is res

Change-Id: Ieb7da9871bec2494c38527eee1266d99e136a264
Reviewed-on: http://review.couchbase.org/37079
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>