added feature to missingValueProducer to signal error

So that returned value is not passed to app, but causes iocenter to
retry request later.

Change-Id: I71a85675645dd05b7d840a0f28955bf20eda960b
Reviewed-on: http://review.couchbase.org/35888
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>