ln: double encrypt errors received from downstream failures

If we're a trampoline node and received an error from downstream that
we can't fully decrypt, we want to double-wrap it for the original
sender. Previously not implemented because we'd only focused on
receives, where there's no possibility of a downstream error.

While proper error handling will be added in a followup, we add the
bare minimum required here for testing.

Author: carlaKC

lightning/src/ln/onion_utils.rs

@@ -2124,6 +2124,10 @@ impl HTLCFailReason {
 				let mut err = err.clone();
 				let hold_time = hold_time.unwrap_or(0);
 
+				if let Some(secondary_shared_secret) = secondary_shared_secret {
+					process_failure_packet(&mut err, secondary_shared_secret, hold_time);
+					crypt_failure_packet(secondary_shared_secret, &mut err);
+				}
 				process_failure_packet(&mut err, incoming_packet_shared_secret, hold_time);
 				crypt_failure_packet(incoming_packet_shared_secret, &mut err);