如何在Rails 4.2自定义验证器中使错误消息可覆盖？

I have a custom validator. I want it to provide a useful default error message. But if the caller - a model - uses the :message argument to override the message, I want that to work. Unfortunately, I seem to be hard-coding my validation message into my custom validator and don't know how to make it more flexible.I have a custom validator. I want it to provide