502, 504 (and maybe 503) `CONNECT` proxy response codes should be treated as though they are direct connection failures

[reivilibre]

offshoot from #15883:

If a 502 or 504 is returned as the status code of the CONNECT verb when proxying, Synapse should treat this as though it tried a connection itself and failed.
It should not log a CRITICAL log line but the same kind of handling as e.g. a connection timeout or refused connection should be used.

We might want to extend support for this to 503 as a kind of quirk given that in #15883 the reporter's proxy software returned 503 instead (this seems incorrect to me but I suppose it'd be worth checking more generally).