MB-6921: File not found returns Not Found instead of NMVB
MB-6921: File not found returns Not Found instead of NMVB

Since we visit an item in the memory hashtable we know that the
vbucket exists so if a file doesn't exist or the file is empty
then we should return NOT FOUND.

Change-Id: I9dd20e044f810702ff2af9c1a807c8a408427941
Reviewed-on: http://review.couchbase.org/21651
Reviewed-by: Jin Lim <c45cd80a556f432a3607171a6818cd5a074b5cd4@couchbase.com>
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/ep-engine,teligent-ru/ep-engine,daverigby/ep-engine,jimwwalker/ep-engine,daverigby/kv_engine,jimwwalker/ep-engine,abhinavdangeti/ep-engine,jimwwalker/ep-engine,teligent-ru/ep-engine,hisundar/ep-engine,owendCB/ep-engine,daverigby/ep-engine,couchbase/ep-engine,owendCB/ep-engine,sriganes/ep-engine,jimwwalker/ep-engine,membase/ep-engine,daverigby/kv_engine,owendCB/ep-engine,sriganes/ep-engine,daverigby/ep-engine,couchbase/ep-engine,abhinavdangeti/ep-engine,membase/ep-engine,hisundar/ep-engine,abhinavdangeti/ep-engine,abhinavdangeti/ep-engine,membase/ep-engine,couchbase/ep-engine,sriganes/ep-engine,owendCB/ep-engine,couchbase/ep-engine,sriganes/ep-engine,abhinavdangeti/ep-engine,hisundar/ep-engine,membase/ep-engine,teligent-ru/ep-engine,daverigby/kv_engine,teligent-ru/ep-engine,daverigby/kv_engine,hisundar/ep-engine