There you go:
https://github.com/racker/reach/blob/master/cloudkick/webapp/site_media/js/ck/validators/ko_validator_wrapper.js?source=c#L21
https://github.com/racker/reach/blob/master/cloudkick/webapp/site_media/js/ck/validators/validation_result.js?source=cc#L10

The message is even optional there.