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)
this error message is duplicated, we should extract it.

extract this code https://github.com/caelum/vraptor4/blob/master/vraptor-core/src/main/java/br/com/caelum/vraptor/observer/ExecuteMethod.java#L87-L102

to a method like: `messages.assertAbsenceOfErrors()` and call this method on both here and on ExecuteMethod.

better names? @renanigt @Turini @garcia-jj