Map exceptions to different response codes.

I have an app that uses a nice url scheme based on product names, so bad URLs are going to happen. There are several actions that require a valid thing but it may not exist. A requirement is to display a nice page, but return a 404. It would be great if I could just throw a MissingThingException and map that to a particular controller/action and have it generate a 404 response.

A possible URL mapping syntax would be...

"404"(controller: "missing", action: "thing", exception: MissingThingException)

The presence of the exception key prevents this from being used as a global mapping for 404s. This should be open for any valid HTTP response code.