clientv3: only return closing error to watcher if context is not canceled
clientv3: only return closing error to watcher if context is not canceled

Fixes #6503

zaphod-concur/etcd,yudai/etcd,heyitsanthony/etcd,vimalk78/etcd,nekto0n/etcd,cockroachdb/etcd,visheshnp/etcd,xiang90/etcd,ravigadde/etcd,etcd-io/etcd,fanminshi/etcd,plasticbox/etcd,cockroachdb/etcd,joshix/etcd,akauppi/etcd,rchicoli/etcd,mkumatag/etcd,mkumatag/etcd,jlamillan/etcd,rnaveiras/etcd,sebrandon1/etcd,gyuho/etcd,mqliang/etcd,OyTao/etcd_learning,insequent/etcd,fanminshi/etcd,coreos/etcd,li-ang/etcd,joshix/etcd,tguangch/etcd,a-robinson/etcd,sinsharat/etcd,bdarnell/etcd,endocode/etcd,whuwxl/etcd,armstrongli/etcd,luxas/etcd,qqsun8819/etcd,sinsharat/etcd,endocode/etcd,visheshnp/etcd,mickep76/etcd,handsomesun/etcd,heyitsanthony/etcd,lorneli/etcd,glevand/coreos--etcd,bluebreezecf/etcd,lumjjb/etcd,bdarnell/etcd,mqliang/etcd,a-robinson/etcd,armstrongli/etcd,qqsun8819/etcd,mkumatag/etcd,li-ang/etcd,hongchaodeng/etcd,akauppi/etcd,OyTao/etcd_learning,handsomesun/etcd,CrazyJvm/etcd,bluebreezecf/etcd,rchicoli/etcd,sebrandon1/etcd,rnaveiras/etcd,etcd-io/etcd,tguangch/etcd,nekto0n/etcd,lumjjb/etcd,insequent/etcd,lorneli/etcd,dannysauer/etcd,xiang90/etcd,YuleiXiao/etcd,gyuho/etcd,mickep76/etcd,whuwxl/etcd,plasticbox/etcd,hongchaodeng/etcd,zaphod-concur/etcd,jlamillan/etcd,rnaveiras/etcd,ajityagaty/etcd,yudai/etcd,ravigadde/etcd,dannysauer/etcd,mqliang/etcd,coreos/etcd,YuleiXiao/etcd,glevand/coreos--etcd,luxas/etcd,ajityagaty/etcd,ajityagaty/etcd,CrazyJvm/etcd,etcd-io/etcd,vimalk78/etcd