CBQE-0: print key value if ValidateDataTask failed

Change-Id: Ia07c5c7da9ef57463af9e64d028ef99ca7ae8df4
Reviewed-on: http://review.couchbase.org/33702
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>