[feature]: Add distinct error code for keysend rejection

[myxmaster]

Currently, when a keysend payment fails because the receiving node doesn't accept keysend payments, LND returns FAILURE_REASON_INCORRECT_PAYMENT_DETAILS.
-> It would be nice to see a specific error code like FAILURE_REASON_KEYSEND_NOT_ACCEPTED in this case.

[ziggie1984]

Because Keysends are only a Specification Enhancement not part of the mayor BOLT Spec. this should be added to the BLIP so all implementation will follow the same spec : https://github.com/lightning/blips/blob/master/blip-0003.md

Regarding the requested feature, the lightning specification already allows for additional error details as part of a TLV record added to the failuremsg (https://github.com/lightning/bolts/blob/master/04-onion-routing.md#failure-messages). So in general I also endorse this idea.

[myxmaster]

#6414 (comment):
"Note that since many nodes out there currently support keysend without signalling for the feature bit"
-> Is this still the case? If not, wallets could simply check if dest node accepts keysend before trying to send.

I mean, bLIP-3 clearly says: "SHOULD only send payments to nodes advertising the keysend feature bit"

[ziggie1984]

that's a workaround if the wallet implementation does the pathfinding and has the node-graph available, but I think it still makes sense to have the error reporting for these scenarios.