Correctly return ENGINE_ERROR_CODE in processing mutation messages

Change-Id: I72cadd9beb9a024be7fab5d54c7ecf8a85a40c79
Reviewed-on: http://review.couchbase.org/40653
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>