Redirect to candidate page on failed validation. Fixes #157.

# Changes
 - Redirects to the candidate page on failed validation. This way users can fix their form input without having to scroll and find their candidate in the category list again.

# Notes
This is real messy. Form Requests in Laravel 5 will make this a lot cleaner to handle, since each form request can have its own `response()` method to customize what happens on a failed validation. Until then... :cactus: