AuthenticateAttribute tweaks

* Fix relative redirect
* Improve error message

The error message in AuthorizationAttribute was confusing me for a while: providers are more than just OAuth.

BTW, it seems to me that `PathUtility.CombineWith` is broken - it is impossible to combine slash with anything:

    "/".CombineWith("/some/other/path")

returns `some/other/path` - is that intended result?