figure out which federation errors should cause backoff

[richvdh]

currently the federation sender will back off sending transactions to a host which returns:

• a 50x error
• a 401
• a 429
• a 404, but only for certain endpoints

(see https://github.com/matrix-org/synapse/blob/v1.26.0/synapse/util/retryutils.py#L176-L183)

This seems... arbitrary. For example, a homeserver which has been configured with a federation whitelist will return a 403 to any homeserver not on that whitelist. Surely we should back off in that case.

[richvdh]

dup #5442