Add new Bolt12PaymentError for failed blinded path creation.

Currently used when initiating an async payment via held_htlc_available OM. This
OM needs a reply path back to us, so use this error for our invoice_error OM if
we fail to create said reply path.

lightning/src/ln/channelmanager.rs

@@ -4347,7 +4347,7 @@ where
 				Ok(paths) => paths,
 				Err(()) => {
 					self.abandon_payment_with_reason(payment_id, PaymentFailureReason::RouteNotFound);
-					res = Err(Bolt12PaymentError::SendingFailed(RetryableSendFailure::RouteNotFound));
+					res = Err(Bolt12PaymentError::BlindedPathCreationFailed);
 					return NotifyOption::DoPersist
 				}
 			};
@@ -10931,6 +10931,12 @@ where
 						log_trace!($logger, "Failed paying invoice: {:?}", e);
 						InvoiceError::from_string(format!("{:?}", e))
 					},
+					#[cfg(async_payments)]
+					Err(Bolt12PaymentError::BlindedPathCreationFailed) => {
+						let err_msg = "Failed to create a blinded path back to ourselves";
+						log_trace!($logger, "{}", err_msg);
+						InvoiceError::from_string(err_msg.to_string())
+					},
 					Err(Bolt12PaymentError::UnexpectedInvoice)
 						| Err(Bolt12PaymentError::DuplicateInvoice)
 						| Ok(()) => return None,

lightning/src/ln/outbound_payment.rs

@@ -523,6 +523,15 @@ pub enum Bolt12PaymentError {
 	UnknownRequiredFeatures,
 	/// The invoice was valid for the corresponding [`PaymentId`], but sending the payment failed.
 	SendingFailed(RetryableSendFailure),
+	#[cfg(async_payments)]
+	/// Failed to create a blinded path back to ourselves.
+	///
+	/// We attempted to initiate payment to a [`StaticInvoice`] but failed to create a reply path for
+	/// our [`HeldHtlcAvailable`] message.
+	///
+	/// [`StaticInvoice`]: crate::offers::static_invoice::StaticInvoice
+	/// [`HeldHtlcAvailable`]: crate::onion_message::async_payments::HeldHtlcAvailable
+	BlindedPathCreationFailed,
 }
 
 /// Indicates that we failed to send a payment probe. Further errors may be surfaced later via