MB-3370 Check if db files exist before restoring
MB-3370 Check if db files exist before restoring

We can give more accurate error messages if any db file doesn't
exist.

Change-Id: I84226492f977b46094b2755bf176eeb80567d3fd
Reviewed-on: http://review.membase.org/6511
Tested-by: Bin Cui <ed18693fff32c00e22495f4877a3b901bed09041@gmail.com>
Reviewed-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>

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