[#1442] cli/tree: Enchance error message if rpc-endpoint isn't defined

Signed-off-by: Aleksey Savchuk <a.savchuk@yadro.com>

cmd/frostfs-cli/modules/tree/client.go

@@ -2,6 +2,7 @@ package tree
 
 import (
 	"context"
+	"fmt"
 	"strings"
 
 	"git.frostfs.info/TrueCloudLab/frostfs-node/cmd/frostfs-cli/internal/common"
@@ -20,7 +21,13 @@ import (
 // after making Tree API public.
 func _client() (tree.TreeServiceClient, error) {
 	var netAddr network.Address
-	err := netAddr.FromString(viper.GetString(commonflags.RPC))
+
+	rpcEndpoint := viper.GetString(commonflags.RPC)
+	if rpcEndpoint == "" {
+		return nil, fmt.Errorf("%s is not defined", commonflags.RPC)
+	}
+
+	err := netAddr.FromString(rpcEndpoint)
 	if err != nil {
 		return nil, err
 	}