Media download endpoints do not describe a 404 response

[DMRobertson]

Link to problem area:

• https://spec.matrix.org/v1.8/client-server-api/#get_matrixmediav3downloadservernamemediaid
• https://spec.matrix.org/v1.8/client-server-api/#get_matrixmediav3downloadservernamemediaidfilename
• https://spec.matrix.org/v1.8/client-server-api/#get_matrixmediav3thumbnailservernamemediaid

Issue
These don't mention 404 return code, despite this being reasonable and obvious.

Expected behaviour
Synapse returns 404 in some situations: Synapse does this, e.g. https://github.com/matrix-org/synapse/blob/999bd77d3abb7b0a4430f31f5912956c3bc100ee/synapse/media/media_repository.py#L510

so we should update the spec text to reflect this.

[turt2live]

Merging into #1122

[DMRobertson]

Thanks---I should have searched (harder?).

[turt2live]

no worries - it took me several searches 😅

could have sworn we had an issue for it already, but turns out it was for thumbnails only (now fixed)