Add some sugar for common REST errors

This PR adds support for a few REST errors used in scenarios where we are otherwise rolling custom TypedErrors:

- `401`
- `409`
- `410`

In addition, I added `.Forbidden` as a `403` generator. In order to provide back-compat support, `NotAuthorized` still generates a `403`. However, the similarly named `.Unauthorized` throws as a `401`  per [W3C naming](http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html)