Fix 500 error when we get an error handling a PDU

FederationServer doesn't have a send_failure (and nor does its subclass,
ReplicationLayer), so this was failing.

I'm not really sure what the idea behind send_failure is, given (a) we don't do
anything at the other end with it except log it, and (b) we also send back the
failure via the transaction response. I suspect there's a whole lot of dead
code around it, but for now I'm just removing the broken bit.