Add FailureCode enum

This enum is used to specify which error code and data to send
to peers when failing back an HTLC.

Author: alecchendev

lightning/src/ln/channelmanager.rs

@@ -289,6 +289,25 @@ struct ReceiveError {
 	msg: &'static str,
 }
 
+/// This enum is used to specify which error data to send to peers when failing back an HTLC
+/// using [`ChannelManager::fail_htlc_backwards_with_reason`].
+///
+/// For more info on failure codes, see <https://github.com/lightning/bolts/blob/master/04-onion-routing.md#failure-messages>.
+#[derive(Clone, Copy)]
+pub enum FailureCode {
+	/// We had a temporary error processing the payment. Useful if no other error codes fit
+	/// and you want to indicate that the payer may want to retry.
+	TemporaryNodeFailure             = 0x2000 | 2,
+	/// We have a required feature which was not in this onion. For example, you may require
+	/// some additional metadata that was not provided with this payment.
+	RequiredNodeFeatureMissing       = 0x4000 | 0x2000 | 3,
+	/// You may wish to use this when a `payment_preimage` is unknown, or the CLTV expiry of
+	/// the HTLC is too close to the current block height for safe handling.
+	/// Using this failure code in [`ChannelManager::fail_htlc_backwards_with_reason`] is
+	/// equivalent to calling [`ChannelManager::fail_htlc_backwards`].
+	IncorrectOrUnknownPaymentDetails = 0x4000 | 15,
+}
+
 type ShutdownResult = (Option<(OutPoint, ChannelMonitorUpdate)>, Vec<(HTLCSource, PaymentHash, PublicKey, [u8; 32])>);
 
 /// Error type returned across the peer_state mutex boundary. When an Err is generated for a