Return outgoing scid along with HTLC failure message

In order to surface `HTLCHandlingFailed` events for HTLCs that we cannot
forward, we need to track their intended outgoing short channel ID, if
one exists, to provide it as part of the event.

Author: wpaulino

lightning/src/ln/channelmanager.rs

@@ -3162,12 +3162,10 @@ where
 
 	fn decode_update_add_htlc_onion(
 		&self, msg: &msgs::UpdateAddHTLC, counterparty_node_id: &PublicKey, channel: Option<&mut Channel<SP>>,
-	) -> Result<
-		(onion_utils::Hop, [u8; 32], Option<Result<PublicKey, secp256k1::Error>>), HTLCFailureMsg
-	> {
+	) -> Result<(onion_utils::Hop, [u8; 32], Option<Result<PublicKey, secp256k1::Error>>), (HTLCFailureMsg, Option<u64>)> {
 		let (next_hop, shared_secret, next_packet_details_opt) = decode_incoming_update_add_htlc_onion(
 			msg, &self.node_signer, &self.logger, &self.secp_ctx
-		)?;
+		).map_err(|fail_msg| (fail_msg, None))?;
 
 		let next_packet_details = match next_packet_details_opt {
 			Some(next_packet_details) => next_packet_details,
@@ -3185,7 +3183,14 @@ where
 		};
 
 		let fail_msg_from_err = |err_msg: &'static str, err_code: u16, chan_update: Option<msgs::ChannelUpdate>| {
-			self.process_failed_accept_err(msg, counterparty_node_id, err_msg, err_code, chan_update, is_intro_node_forward, &shared_secret)
+			let fail_msg = self.process_failed_accept_err(
+				msg, counterparty_node_id, err_msg, err_code, chan_update, is_intro_node_forward, &shared_secret
+			);
+			let outgoing_scid = match &fail_msg {
+				HTLCFailureMsg::Relay(_) => Some(next_packet_details.outgoing_scid),
+				HTLCFailureMsg::Malformed(_) => None,
+			};
+			(fail_msg, outgoing_scid)
 		};
 
 		let res = if let Some(channel) = channel {
@@ -6679,7 +6684,7 @@ where
 								msg, counterparty_node_id, shared_secret, next_hop,
 								chan.context.config().accept_underpaying_htlcs, next_packet_pk_opt,
 							),
-						Err(e) => PendingHTLCStatus::Fail(e)
+						Err((fail_msg, _)) => PendingHTLCStatus::Fail(fail_msg)
 					};
 					let create_pending_htlc_status = |chan: &Channel<SP>, pending_forward_info: PendingHTLCStatus, error_code: u16| {
 						if msg.blinding_point.is_some() {