client: add IsKeyNotFound function
client: add IsKeyNotFound function

This can be used to check if an error is client.ErrorCodeKeyNotFound.
Related to https://github.com/coreos/etcd/issues/4080.

swingbach/etcd,rnaveiras/etcd,joshix/etcd,mqliang/etcd,mkumatag/etcd,ajityagaty/etcd,jlamillan/etcd,glevand/coreos--etcd,siddontang/etcd,qqsun8819/etcd,a-robinson/etcd,visheshnp/etcd,OyTao/etcd_learning,handsomesun/etcd,raoofm/etcd,chipironcin/etcd,nekto0n/etcd,gyuho/etcd,shawnps/etcd,akauppi/etcd,insequent/etcd,akauppi/etcd,qqsun8819/etcd,li-ang/etcd,cockroachdb/etcd,endocode/etcd,jlamillan/etcd,mqliang/etcd,tamird/etcd,dannysauer/etcd,xiang90/etcd,tamird/etcd,yudai/etcd,westhood/etcd,sofuture/etcd,coreos/etcd,a-robinson/etcd,lorneli/etcd,joshix/etcd,dnaeon/etcd,whuwxl/etcd,mickep76/etcd,jonboulle/etcd,hongchaodeng/etcd,ravigadde/etcd,wantonsolutions/dinv-etcd,zaphod-concur/etcd,YuleiXiao/etcd,lumjjb/etcd,linuxcer/etcd,luxas/etcd,dnaeon/etcd,insequent/etcd,tguangch/etcd,ajityagaty/etcd,linuxcer/etcd,fanminshi/etcd,shawnps/etcd,dannysauer/etcd,raoofm/etcd,YuleiXiao/etcd,bluebreezecf/etcd,armstrongli/etcd,jonboulle/etcd,bdarnell/etcd,ktateish/etcd,wantonsolutions/dinv-etcd,plasticbox/etcd,sofuture/etcd,yudai/etcd,ktateish/etcd,achanda/etcd,CrazyJvm/etcd,coreos/etcd,Timer/etcd,ravigadde/etcd,rnaveiras/etcd,CrazyJvm/etcd,glycerine/etcd,chipironcin/etcd,mkumatag/etcd,purpleidea/etcd,glevand/coreos--etcd,etcd-io/etcd,cockroachdb/etcd,wantonsolutions/dinv-etcd,zaphod-concur/etcd,handsomesun/etcd,tguangch/etcd,li-ang/etcd,Timer/etcd,mqliang/etcd,etcd-io/etcd,StorReduce/etcd,bluebreezecf/etcd,sinsharat/etcd,lorneli/etcd,purpleidea/etcd,StorReduce/etcd,mickep76/etcd,heyitsanthony/etcd,vimalk78/etcd,sebrandon1/etcd,siddontang/etcd,swingbach/etcd,fanminshi/etcd,xiang90/etcd,vimalk78/etcd,sinsharat/etcd,gyuho/etcd,armstrongli/etcd,rchicoli/etcd,plasticbox/etcd,rchicoli/etcd,lumjjb/etcd,bdarnell/etcd,sebrandon1/etcd,visheshnp/etcd,ajityagaty/etcd,achanda/etcd,westhood/etcd,whuwxl/etcd,etcd-io/etcd,nekto0n/etcd,hongchaodeng/etcd,rnaveiras/etcd,mkumatag/etcd,luxas/etcd,OyTao/etcd_learning,heyitsanthony/etcd,endocode/etcd,glycerine/etcd