Add 404 as valid response for /thumbnail and /download when mediaId does not exist #1122
Description
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. |