Error when overpaying an intercepted forward

Author: valentinewallace

lightning/src/ln/channelmanager.rs

@@ -3536,14 +3536,17 @@ where
 				err: format!("Payment with intercept id {} not found", log_bytes!(intercept_id.0))
 			})?;
 
+		let skimmed_fee_msat =
+			payment.forward_info.outgoing_amt_msat.checked_sub(amt_to_forward_msat)
+			.ok_or_else(|| APIError::APIMisuseError {
+				err: format!("Attempted to forward intercept with id {} with amount {} msat, maximum forward amount was {} msat", log_bytes!(intercept_id.0), amt_to_forward_msat, payment.forward_info.outgoing_amt_msat)
+			})?;
 		let routing = match payment.forward_info.routing {
 			PendingHTLCRouting::Forward { onion_packet, .. } => {
 				PendingHTLCRouting::Forward { onion_packet, short_channel_id: next_hop_scid }
 			},
 			_ => unreachable!() // Only `PendingHTLCRouting::Forward`s are intercepted
 		};
-		let skimmed_fee_msat =
-			payment.forward_info.outgoing_amt_msat.saturating_sub(amt_to_forward_msat);
 		let pending_htlc_info = PendingHTLCInfo {
 			skimmed_fee_msat: if skimmed_fee_msat == 0 { None } else { Some(skimmed_fee_msat) },
 			outgoing_amt_msat: amt_to_forward_msat, routing, ..payment.forward_info