Provide more info why we don't have any thumbnails to serve

[MadLittleMods]

Provide more info why we don't have any thumbnails to serve.

Fix #13016

New error code and status

Before

Previously, we returned a 404 for /thumbnail which isn't even in the spec.

{
  "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.

{
    "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: matrix-org/matrix-spec#1122

We can also iterate on the bugs where Synapse doesn't thumbnail when it should in other issues/PRs.

Dev notes

When dynamic_thumbnails enabled:

`/thumbnail`
MediaRepositoryResource
ThumbnailResource
_select_or_generate_local_thumbnail
...

When dynamic_thumbnails disabled:

`/thumbnail`
MediaRepositoryResource
ThumbnailResource
_respond_local_thumbnail
_select_and_respond_with_thumbnail

Pull Request Checklist

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  (run the linters)

[richvdh]

I applaud the effort, though I fear it's going to be tricky to give enough information that it's actually going to be useful for the user.

Some comments, though.

[richvdh]

still think this should be a 404 and M_NOT_FOUND.

[MadLittleMods]

The spec says no right now, matrix-org/matrix-spec#1122

It does make sense to me in some cases:

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.

In the cases I ran into, 400 makes more sense since the media did exist, Synapse just didn't thumbnail it.

[reivilibre]

Thanks for taking the time to try and make this clearer to people

[richvdh]

generally lgtm. A couple of nits.

[richvdh]

thank you! ❤️

[MadLittleMods]

Thanks for the review @richvdh and @reivilibre 🐵