CBQE-0: fix failover test cases - failure due to graceful failure should be captured correctly

Change-Id: I0452134bad7783eaf1f93ef052882404a6d18d8b
Reviewed-on: http://review.couchbase.org/37664
Reviewed-by: Thuan Nguyen <soccon@gmail.com>
Reviewed-by: Parag Agarwal <agarwal.parag@gmail.com>
Tested-by: Parag Agarwal <agarwal.parag@gmail.com>