Throw NotFound when a poster image doesn't exist

[gmemstr]

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[TapGhoul]

Do not merge this. You will break the poster prefabs. VRChat will not load anything unless it's 200/201/anything that's a valid response.
I tested with 204 no content, it will refuse to load even with that. Ergo, vrchat is handling status codes correctly - something we don't want.

If we plan to do a 404, we need to make a change in behavior to the system, including a not found image hard coded that we can't change. But as we don't really want to do that, you must return a 200 at all times.

I suggest, if you want to add tracing in the middle, you add an X-Inner-Status header or similar that you can trace.