Dendrite returns improper error codes for unknown endpoints

[clokep]

Dendrite returns improper responses for unknown endpoints per MSC3743, see matrix-org/complement#565 for tests I'm adding for this.

Dendrite is:

• Returning a 404 instead of a 405 for an unknown methods on Client-Server APIs.
• Returning a text response, instead of JSON, error on /media, /key, /federation and unknown prefixes.

[clokep]

Related: #2740

[clokep]

Tests can probably be re-enabled at https://github.com/matrix-org/complement/blob/778409aa2237e4e2fddb54e3fcdeb4a21d77674d/tests/unknown_endpoints_test.go#L41 😄

[clokep]

And I see you already did this at matrix-org/complement#630 🤦