errorMessage callback

Hey!

I needed a way to be able to better customize the error message shown on a field. I use [i18n.js](https://github.com/fnando/i18n-js) in my app, so I can keep my error messages consistent between the server and the client.

I couldn't find a way to define the error message on the fly. This is what I've come up.

What do you think?

Keith