Expand the return type of handle_onion_message_response.

The return type is expanded to handle three cases:
1. Ok(None) in case of no response to be sent.
2. Ok(Some(SendSuccess) and Err(SendError) in case of successful and
   unsuccessful queueing up of response messages respectively.

This allows the user to get access to the Success/Failure status of the sending
of response and handle it accordingly.

lightning/src/onion_message/functional_tests.rs

@@ -18,7 +18,7 @@ use crate::routing::test_utils::{add_channel, add_or_update_node};
 use crate::sign::{NodeSigner, Recipient};
 use crate::util::ser::{FixedLengthReader, LengthReadable, Writeable, Writer};
 use crate::util::test_utils;
-use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction, SendError};
+use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction, SendError, SendSuccess};
 use super::offers::{OffersMessage, OffersMessageHandler};
 use super::packet::{OnionMessageContents, Packet};
 
@@ -351,7 +351,11 @@ fn async_response_over_one_blinded_hop() {
 	let response_instruction = nodes[0].custom_message_handler.handle_custom_message(message, responder);
 
 	// 6. Simulate Alice asynchronously responding back to Bob with a response.
-	nodes[0].messenger.handle_onion_message_response(response_instruction);
+	assert_eq!(
+		nodes[0].messenger.handle_onion_message_response(response_instruction),
+		Ok(Some(SendSuccess::Buffered)),
+	);
+
 	bob.custom_message_handler.expect_message(TestCustomMessage::Response);
 
 	pass_along_path(&nodes);