Allow custom message for validation errors

[fdonzello]

I think this is the first step to allow localization of error messages.
Actually, from now on, a developer can use the translator instance to translate the custom message error.

[fdonzello]

Any chances? :)

[fdonzello]

hi @markbates, I'd like to contribute to the buffalo project but I need to know why no answers (I am not pretending you to merge or reject) have been posted since Mar 25.

At least a "I don't like your code" or "too busy now.." would have been appreciated...
But so far, I don't know if my efforts would lead to any improvements to the project.

[markbates]

@fdonzello please don’t take offense. This PR just slipped through the cracks. I’ll take a look at it today. Over the last two months I’ve been traveling nearly every other week, so I’ve gotten way behind on some reviews. This is a lesser touched repo so it fell completely off the radar. I’ll get to it today.

[fdonzello]

no offense at all ;)

[stanislas-m]

@fdonzello Can you add an example about that feature in the Readme? :)

[fdonzello]

sure, I'll provide asap