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

@@ -351,7 +351,7 @@ 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);
+	let _ = nodes[0].messenger.handle_onion_message_response(response_instruction);
 	bob.custom_message_handler.expect_message(TestCustomMessage::Response);
 
 	pass_along_path(&nodes);

lightning/src/onion_message/messenger.rs

@@ -1009,9 +1009,9 @@ where
 
 	/// Handles the response to an [`OnionMessage`] based on its [`ResponseInstruction`],
 	/// enqueueing any response for sending.
-	pub fn handle_onion_message_response<T: OnionMessageContents>(
+	pub fn handle_onion_message_response<T: OnionMessageContents> (
 		&self, response: ResponseInstruction<T>
-	) {
+	) -> Result<Option<SendSuccess>, SendError> {
 		let (response, reply_path) = match response {
 			ResponseInstruction::WithReplyPath(response) => (response, self.create_blinded_path().ok()),
 			ResponseInstruction::WithoutReplyPath(response) => (response, None),
@@ -1020,18 +1020,23 @@ where
 					"Missing reply path when responding to {} onion message",
 					T::msg_type()
 				);
-				return
+				return Ok(None)
 			}
 		};
 
-		let _ = self.find_path_and_enqueue_onion_message(
+		let res = self.find_path_and_enqueue_onion_message(
 			response.message, Destination::BlindedPath(response.reply_path), reply_path,
 			format_args!(
 				"when responding to {} onion message with path_id {:02x?}",
 				T::msg_type(),
 				response.path_id
 			)
 		);
+
+		match res {
+			Ok(result) => Ok(Some(result)),
+			Err(result) => Err(result)
+		}
 	}
 
 	#[cfg(test)]
@@ -1118,14 +1123,14 @@ where
 							|reply_path| Responder::new(reply_path, path_id)
 						);
 						let response_instructions = self.offers_handler.handle_message(msg, responder);
-						self.handle_onion_message_response(response_instructions);
+						let _ = self.handle_onion_message_response(response_instructions);
 					},
 					ParsedOnionMessageContents::Custom(msg) => {
 						let responder = reply_path.map(
 							|reply_path| Responder::new(reply_path, path_id)
 						);
 						let response_instructions = self.custom_handler.handle_custom_message(msg, responder);
-						self.handle_onion_message_response(response_instructions);
+						let _ = self.handle_onion_message_response(response_instructions);
 					},
 				}
 			},