etcdctl: fix get command error when no arg provided
etcdctl: fix get command error when no arg provided

The current error is incorrect. Providing a similar one to the del
command.

joshix/etcd,rnaveiras/etcd,etcd-io/etcd,li-ang/etcd,mqliang/etcd,mqliang/etcd,xiang90/etcd,gyuho/etcd,cockroachdb/etcd,joshix/etcd,dannysauer/etcd,mqliang/etcd,sinsharat/etcd,vimalk78/etcd,ajityagaty/etcd,coreos/etcd,vimalk78/etcd,gyuho/etcd,etcd-io/etcd,ajityagaty/etcd,cockroachdb/etcd,rnaveiras/etcd,xiang90/etcd,li-ang/etcd,etcd-io/etcd,rnaveiras/etcd,coreos/etcd,mkumatag/etcd,mkumatag/etcd,dannysauer/etcd,sinsharat/etcd,mkumatag/etcd,ajityagaty/etcd