clientv3: define error type for closed watcher
clientv3: define error type for closed watcher

visheshnp/etcd,dannysauer/etcd,YuleiXiao/etcd,xiang90/etcd,dannysauer/etcd,armstrongli/etcd,lorneli/etcd,vimalk78/etcd,mkumatag/etcd,jlamillan/etcd,bluebreezecf/etcd,visheshnp/etcd,etcd-io/etcd,handsomesun/etcd,whuwxl/etcd,heyitsanthony/etcd,plasticbox/etcd,YuleiXiao/etcd,cockroachdb/etcd,lumjjb/etcd,yudai/etcd,etcd-io/etcd,li-ang/etcd,mkumatag/etcd,whuwxl/etcd,sebrandon1/etcd,tguangch/etcd,rnaveiras/etcd,plasticbox/etcd,handsomesun/etcd,li-ang/etcd,fanminshi/etcd,lumjjb/etcd,bdarnell/etcd,coreos/etcd,ajityagaty/etcd,mqliang/etcd,joshix/etcd,a-robinson/etcd,mqliang/etcd,rnaveiras/etcd,mqliang/etcd,vimalk78/etcd,a-robinson/etcd,sinsharat/etcd,sebrandon1/etcd,mkumatag/etcd,gyuho/etcd,fanminshi/etcd,jlamillan/etcd,bdarnell/etcd,ajityagaty/etcd,armstrongli/etcd,ajityagaty/etcd,heyitsanthony/etcd,glevand/coreos--etcd,coreos/etcd,sinsharat/etcd,cockroachdb/etcd,glevand/coreos--etcd,mickep76/etcd,lorneli/etcd,xiang90/etcd,yudai/etcd,rnaveiras/etcd,tguangch/etcd,etcd-io/etcd,joshix/etcd,gyuho/etcd,mickep76/etcd,bluebreezecf/etcd,akauppi/etcd,akauppi/etcd