ln: add trampoline LocalHTLCFailureReason variants per spec

This commit adds three new local htlc failure error reasons:
`TemporaryTrampolineFailure`, `TrampolineFeeOrExpiryInsufficient`,
and `UnknownNextTrampoline` for trampoline payment forwarding failures.

Author: a-mpch

lightning/src/ln/onion_utils.rs

@@ -1663,6 +1663,13 @@ pub enum LocalHTLCFailureReason {
 	PeerOffline,
 	/// The HTLC was failed because the channel balance was overdrawn.
 	ChannelBalanceOverdrawn,
+	/// We have been unable to forward a payment to the next Trampoline node but may be able to
+	/// do it later.
+	TemporaryTrampolineFailure,
+	/// The amount or CLTV expiry were insufficient to route the payment to the next Trampoline.
+	TrampolineFeeOrExpiryInsufficient,
+	/// The specified next Trampoline node cannot be reached from our node.
+	UnknownNextTrampoline,
 }
 
 impl LocalHTLCFailureReason {
@@ -1704,6 +1711,9 @@ impl LocalHTLCFailureReason {
 			Self::InvalidOnionPayload | Self::InvalidTrampolinePayload => PERM | 22,
 			Self::MPPTimeout => 23,
 			Self::InvalidOnionBlinding => BADONION | PERM | 24,
+			Self::TemporaryTrampolineFailure => NODE | 25,
+			Self::TrampolineFeeOrExpiryInsufficient => NODE | 26,
+			Self::UnknownNextTrampoline => PERM | 27,
 			Self::UnknownFailureCode { code } => *code,
 		}
 	}
@@ -1863,7 +1873,10 @@ ser_failure_reasons!(
 	(39, HTLCMinimum),
 	(40, HTLCMaximum),
 	(41, PeerOffline),
-	(42, ChannelBalanceOverdrawn)
+	(42, ChannelBalanceOverdrawn),
+	(43, TemporaryTrampolineFailure),
+	(44, TrampolineFeeOrExpiryInsufficient),
+	(45, UnknownNextTrampoline)
 );
 
 impl From<&HTLCFailReason> for HTLCHandlingFailureReason {
@@ -2031,6 +2044,11 @@ impl HTLCFailReason {
 					debug_assert!(false, "Unknown failure code: {}", code)
 				}
 			},
+			LocalHTLCFailureReason::TemporaryTrampolineFailure => debug_assert!(data.is_empty()),
+			LocalHTLCFailureReason::TrampolineFeeOrExpiryInsufficient => {
+				debug_assert_eq!(data.len(), 10)
+			},
+			LocalHTLCFailureReason::UnknownNextTrampoline => debug_assert!(data.is_empty()),
 		}
 
 		Self(HTLCFailReasonRepr::Reason { data, failure_reason })