Simplify rescue code

By using the same interface for validators and rescue handlers, the code is much simplified.

ping @tjoyal @sgnr 
[here](https://github.com/Shopify/activerecord-rescue_from_duplicate/pull/5/files#diff-f6bc006c0badfd2dd8a0672ce44dc3a9R68) and [here](https://github.com/Shopify/activerecord-rescue_from_duplicate/pull/5/files#diff-f6bc006c0badfd2dd8a0672ce44dc3a9R74)