don't run without command

cmd/oplcli.go

@@ -26,7 +26,7 @@ type args struct {
 
 func Run() {
 	var args args
-	arg.MustParse(&args)
+	p := arg.MustParse(&args)
 
 	switch {
 	case args.Scan != nil:
@@ -35,5 +35,7 @@ func Run() {
 		oplcli.Rename(args.Rename.Files, oplcli.Config{})
 	case args.Art != nil:
 		oplcli.DownloadGameArt(args.Art.Files, args.Art.OutputDirectory, oplcli.Config{})
+	default:
+		p.Fail("Must specify command")
 	}
 }