etcdctl/command: fix shawdowed error variable
etcdctl/command: fix shawdowed error variable

This fixes https://github.com/coreos/etcd/issues/3954.

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