[feature]: multi RFQ: indicate what nodes were offline and no quote could be obtained

[ZZiigguurraatt]

When doing a multi RFQ SendPayment it streams a separate response for each PeerAcceptedSellQuote. This is nice. AddInvoice should too (#1565). It would also be nice to see what peers rejected the quote (#1569).

A third thing that it would be nice to see would be what peers are offline and no quote could be obtained.

Right now, both AddInvoice and SendPayment quickly skip offline nodes but don't stream any indication that the nodes were offline. It is good that they are quickly skipped if unresponsive and don't delay like #1571 , but we should be told what happened in the streaming RPC response.