Add Bolt12PaymentError::SendingFailed variant

Instead of returning Ok when path finding fails, allow returning a
RetryableSendFailure from send_payment_for_bolt12_invoice. Follow up
commits will return such failures.

Author: jkczyz

lightning/src/ln/max_payment_path_len_tests.rs

@@ -388,7 +388,7 @@ fn bolt12_invoice_too_large_blinded_paths() {
 	let invoice_om = nodes[1].onion_messenger.next_onion_message_for_peer(nodes[0].node.get_our_node_id()).unwrap();
 	nodes[0].onion_messenger.handle_onion_message(&nodes[1].node.get_our_node_id(), &invoice_om);
 	// TODO: assert on the invoice error once we support replying to invoice OMs with failure info
-	nodes[0].logger.assert_log_contains("lightning::ln::channelmanager", "Failed paying invoice: OnionPacketSizeExceeded", 1);
+	nodes[0].logger.assert_log_contains("lightning::ln::channelmanager", "Failed paying invoice: SendingFailed(OnionPacketSizeExceeded)", 1);
 
 	let events = nodes[0].node.get_and_clear_pending_events();
 	assert_eq!(events.len(), 1);

lightning/src/ln/outbound_payment.rs

@@ -510,11 +510,8 @@ pub enum Bolt12PaymentError {
 	UnexpectedInvoice,
 	/// Payment for an invoice with the corresponding [`PaymentId`] was already initiated.
 	DuplicateInvoice,
-	/// The [`BlindedPath`]s provided are too large and caused us to exceed the maximum onion hop data
-	/// size of 1300 bytes.
-	///
-	/// [`BlindedPath`]: crate::blinded_path::BlindedPath
-	OnionPacketSizeExceeded,
+	/// The invoice was valid for the corresponding [`PaymentId`], but sending the payment failed.
+	SendingFailed(RetryableSendFailure),
 }
 
 /// Indicates that we failed to send a payment probe. Further errors may be surfaced later via
@@ -848,7 +845,7 @@ impl OutboundPayments {
 			.map_err(|()| {
 				log_error!(logger, "Can't construct an onion packet without exceeding 1300-byte onion \
 					hop_data length for payment with id {} and hash {}", payment_id, payment_hash);
-				Bolt12PaymentError::OnionPacketSizeExceeded
+				Bolt12PaymentError::SendingFailed(RetryableSendFailure::OnionPacketSizeExceeded)
 			})?;
 
 		if let Some(max_fee_msat) = max_total_routing_fee_msat {