Use error message of failed to find a healthy node for karmada-etcd for can not found note to scheduled etcd

Signed-off-by: Lan Liang <gcslyp@gmail.com>

pkg/karmadactl/cmdinit/kubernetes/node.go

@@ -3,6 +3,7 @@ package kubernetes
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"strings"
 
@@ -108,6 +109,10 @@ func (i *CommandInitOption) AddNodeSelectorLabels() error {
 		}
 	}
 
+	if etcdSelectorLabels == nil {
+		return errors.New("failed to find a healthy node for karmada-etcd")
+	}
+
 	etcdSelectorLabels["karmada.io/etcd"] = ""
 	patchData := map[string]interface{}{"metadata": map[string]map[string]string{"labels": etcdSelectorLabels}}
 	playLoadBytes, _ := json.Marshal(patchData)