Show error if haxe-modular fails

[mastef]

This silent death here is deadly, as ( at least in my environment ) it doesn't output anything from Sys.command, and there's no error message :

Maybe something like :

		//Sys.println(cmd + ' ' + args.join(' '));
		var code = Sys.command(cmd, args);
		if (code != 0) {
			Sys.println("[haxe-split] Error when executing command : " + code);
			Sys.println(cmd + ' ' + args.join(' '));
			Sys.exit(code);
		}