send error to stderr

main.go

@@ -20,6 +20,7 @@ func main() {
 	case "run_command":
 		commands.RunCommand()
 	default: 
-		fmt.Println("Command not found: %s\n", os.Args[1])
+		fmt.Fprintf(os.Stderr, "Command not found: %s\n", os.Args[1])
+		os.Exit(1)
 	}
 }