404 clarification

- 404 is overloaded. The new error-handling section defines 404 as "discovery endpoint not present", while the unchanged text further down and the OpenAPI spec define a `/discovery` 404 as "TEI unknown". A client can't tell "fail over to the next endpoint" from "stop" -> I would recommend sticking with TEI unknown. Otherwise, let's discuss.

Mentioned by @taleodor in #261

Compare the new error-handling section, the unchanged text later in the specification, and the OpenAPI definition for the /discovery 404 response; review the context in #261. Confirm whether 404 means that the TEI is unknown or that the discovery endpoint is absent, then update the affected specification text so clients can distinguish failover from stopping.

Written by the indexing model from the issue text.