V2 Sender never reads and parses error response

The V2 sender has no pub interface to read and action on a replyable error supplied by the receiver. If a sender parses a error response instead of a PSBT proposal, they should log a fatal event, close the session, (and try to broadcast the fallback(?)).

I think a fallback broadcast attempt is up to the implementation. Sometimes you want to, sometimes you may not. but we must demo fallback broadcast in payjoin-cli reference.