在cp中使用被黑客入侵的客户端时的Multimc代码255

i have made a hacked client in mcp 1.8.8 and I want to test it in a real instance Minecraft was annoying enter multi mc and the log it here and it says exited with error code 255 I'm not sure how to deciefer it I'm a code not a hacker guy so I hope you guys can help

and some code to fool stack with the pastebin