MB-8903: Better stat handling

The stat response weren't correctly handled in the error case.
Neither on the consumer side (couch_upr), nor on the producer
side (couch_upr_fake_server).

Change-Id: I23dc0f2468b46f3ece65b9a9ee60f475f2306ac0
Reviewed-on: http://review.couchbase.org/32870
Tested-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pratap Chakravarthy <pratapsrc@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>