How to configure a custom url for unrecoverable errors like unsupported providers?
Issue #166 briefly discusses how unrecoverable errors are redirected to the origin with a query string parameter, and not even the values in `defaults` are loaded.  In my case the root endpoint is already busy for legacy reasons, and it's not really acceptable to add oauth error handling as a special case there.  For the time being I'm using a small custom middleware in front of grant to pre-verify that the provider is one I support, but that seems like a bit of a hack, and I'll never be sure I've caught all the reasons it might error.  How can I redirect the "missing provider" error (and any other unrecoverable errors) to a custom endpoint?