Change 400 to a 404

- If you try to delete a data-set and it doesn't exist, return a 404
- 400 Bad Request seemed wrong, because it's a perfectly well formed request, just trying to delete something that doesn't exist.
- This is a really silly semantics thing, and I couldn't find any reliable information on it really, but I did go to the wwws http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html