clientv3: convert errors to rpctypes on returning

For https://github.com/coreos/etcd/issues/5211.

clientv3/client_test.go

@@ -55,16 +55,16 @@ func TestDialTimeout(t *testing.T) {
 	}
 }
 
-func TestIsHalted(t *testing.T) {
-	if !isHalted(nil, fmt.Errorf("etcdserver: some etcdserver error")) {
+func TestIsHaltErr(t *testing.T) {
+	if !isHaltErr(nil, fmt.Errorf("etcdserver: some etcdserver error")) {
 		t.Errorf(`error prefixed with "etcdserver: " should be Halted`)
 	}
 	ctx, cancel := context.WithCancel(context.TODO())
-	if isHalted(ctx, nil) {
+	if isHaltErr(ctx, nil) {
 		t.Errorf("no error and active context should not be Halted")
 	}
 	cancel()
-	if !isHalted(ctx, nil) {
+	if !isHaltErr(ctx, nil) {
 		t.Errorf("cancel on context should be Halted")
 	}
 }