Fixing problem: validation errors ignored on redirects. Closes #476

In conversation on Issue #476, @lucascs asked me to send this PR to start the bug fix.

Here the gist with the problematic controller as he asked me: [Problematic Controller](https://gist.github.com/renanigt/6f88e6fced50bce47ac9)
I did this, but I was expecting a review. @lucascs suggested the `handleErrors` method to turn `unhandledErrors` as  false and return `getErrors`. As you can see here:
https://github.com/renanigt/vraptor4/blob/validationIgnoredOnRedirects/vraptor-core/src/main/java/br/com/caelum/vraptor/validator/Messages.java#L89

Would be a good idea return `void` and just turn `unhandledErrors` to false ?

Thanks @Turini.