FederationDeniedError is not a SynapseError

[MadLittleMods]

FederationDeniedError is not a SynapseError.

Basically the same reasoning that @erikjohnston gave in #13816 (comment)

I'm not sure it makes sense for this to inherit from SynapseError. The risk is that this manages to bubble all the way to the client API, returning a 403, which seems entirely wrong.

See #14191 where SynapseError is given more context and clarified.

Todo

• Make sure any upstream callers handle FederationDeniedError properly and not just the inherited SynapseError class

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Pull request includes a sign off
• Code style is correct
  (run the linters)