Add `404` as valid response for `/thumbnail` and `/download` when `mediaId` does not exist

`400` makes a lot of sense for all of the scenarios where the media repository can't provide a thumbnail for whatever reason when the media exists.

But `404` probably makes more sense when `mediaId` isn't valid and doesn't exist.

## Current spec

https://spec.matrix.org/v1.1/client-server-api/#get_matrixmediav3thumbnailservernamemediaid

**Responses**
Status | Description
--- | ---
200 | A thumbnail of the requested content.
400 | The request does not make sense to the server, or the server cannot thumbnail the content. For example, the client requested non-integer dimensions or asked for negatively-sized images.
413 | The local content is too large for the server to thumbnail.
429 | This request was rate-limited.
502 | The remote content is too large for the server to thumbnail.

Start with the linked Matrix client-server API specification section and locate the response tables for /thumbnail and /download. Add 404 for the case where mediaId does not exist, while preserving the existing responses for other failures. Done means both endpoint specifications clearly list 404 with that condition.

Written by the indexing model from the issue text.