/_matrix/client/v1/media/preview_url should clarify what status codes / errors may be returned

[Half-Shot]

Link to problem area: https://spec.matrix.org/v1.17/client-server-api/#get_matrixclientv1mediapreview_url

Issue

Currently it only documents 200, and 429. However, Synapse returns a 500 error in pretty much all cases except DNS resolution, which is a 502.

Realistically I'd expect a 404 error if the homeserver cannot resolve a URL, with perhaps more specific error codes depending on the type of failure.