`FederationDeniedError` is not a `SynapseError`

Spawning from https://github.com/matrix-org/synapse/pull/13816#discussion_r993262622

synapse/api/errors.py

@@ -248,7 +248,7 @@ class UserDeactivatedError(SynapseError):
         )
 
 
-class FederationDeniedError(SynapseError):
+class FederationDeniedError(RuntimeError):
     """An error raised when the server tries to federate with a server which
     is not on its federation whitelist.