Fix https://github.com/matrix-org/synapse/issues/13016
## New error code and status
### Before
Previously, we returned a `404` for `/thumbnail` which isn't even in the spec.
```json
{
"errcode": "M_NOT_FOUND",
"error": "Not found [b'hs1', b'tefQeZhmVxoiBfuFQUKRzJxc']"
}
```
### After
What does the spec say?
> 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.
>
> *-- https://spec.matrix.org/v1.1/client-server-api/#get_matrixmediav3thumbnailservernamemediaid*
Now with this PR, we respond with a `400` when we don't have thumbnails to serve and we explain why we might not have any thumbnails.
```json
{
"errcode": "M_UNKNOWN",
"error": "Cannot find any thumbnails for the requested media ([b'example.com', b'12345']). This might mean the media is not a supported_media_format=(image/jpeg, image/jpg, image/webp, image/gif, image/png) or that thumbnailing failed for some other reason. (Dynamic thumbnails are disabled on this server.)",
}
```
> Cannot find any thumbnails for the requested media ([b'example.com', b'12345']). This might mean the media is not a supported_media_format=(image/jpeg, image/jpg, image/webp, image/gif, image/png) or that thumbnailing failed for some other reason. (Dynamic thumbnails are disabled on this server.)
---
We still respond with a 404 in many other places. But we can iterate on those later and maybe keep some in some specific places after spec updates/clarification: https://github.com/matrix-org/matrix-spec/issues/1122
We can also iterate on the bugs where Synapse doesn't thumbnail when it should in other issues/PRs.
