refactor: centralise app name validation

cli/app/cp.go

@@ -2,7 +2,6 @@ package app
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"os"
 	"strings"
@@ -23,10 +22,7 @@ var appCpCommand = &cli.Command{
 	ArgsUsage: "<src> <dst>",
 	Usage:     "Copy files to/from a running app service",
 	Action: func(c *cli.Context) error {
-		appName := c.Args().First()
-		if appName == "" {
-			internal.ShowSubcommandHelpAndError(c, errors.New("no app name provided"))
-		}
+		appName := internal.ValidateAppNameArg(c)
 
 		src := c.Args().Get(1)
 		dst := c.Args().Get(2)