Exception Response Codes

I've put together bobbrez/errorspike as an example. By just installing BetterErrors, any error response (i.e. ActiveRecord::RecordNotFound 404) are being set to 500 responses. This makes it difficult to debug asynchronous requests that expect proper error codes.

If `ActionDispatch::ExceptionWrapper` is available, then use it to generate the exception code, otherwise continue issuing a 500.