Return dummy-hop-specific error on blinded-forward underflow

When amount or CLTV validation underflows on a dummy hop, return a
dummy-hop-specific error. This avoids misleading logs and makes
blinded-path failures easier to debug.

Author: shaavan

lightning/src/ln/onion_payment.rs

@@ -673,7 +673,7 @@ pub(super) fn decode_incoming_update_add_htlc_onion<NS: NodeSigner, L: Logger, T
 			) {
 				Ok((amt, cltv)) => (amt, cltv),
 				Err(()) => {
-					return encode_relay_error("Underflow calculating outbound amount or cltv value for blinded forward",
+					return encode_relay_error("Underflow calculating outbound amount or cltv value for dummy hop",
 						LocalHTLCFailureReason::InvalidOnionBlinding, shared_secret.secret_bytes(), None, &[0; 32]);
 				}
 			};