WebID 404 Not Found

Hey,

what is supposed to happen when the WebID URI extracted from the client
certificate returns 404 Not Found when dereferenced?

Step 6.1 in Authentication Sequence [1] talks about fetching WebID profile
by dereferencing it, but does not consider a failure scenario.

What kind of response status code should be sent when such failure occurs?
It is a client error, so probably a 4xx like 400 Bad Request. Although some
servers send SSL-specific like 495 SSL Certificate Error or 496 SSL
Certificate Required, but those are non-standard [2].

[1]
https://www.w3.org/2005/Incubator/webid/spec/tls/#authentication-sequence
[2] https://en.wikipedia.org/wiki/List_of_HTTP_status_codes#nginx