MB-9715: Fixup UPR step

Check the result value from the message producers and
propagate errors back to the core, and return
the correct return value to avoid spinning and
eating up the CPU.

Change-Id: I360d014884425a62e8e8ec1ae60614ceaf8a6ccb
Reviewed-on: http://review.couchbase.org/31121
Reviewed-by: David Liao <david.liao@couchbase.com>
Tested-by: David Liao <david.liao@couchbase.com>