rpc module: simplify error handling in subscriptions

[niklasad1]

Currently, jsonrpsee requires the subscription async block to return an error which is just used to log errors.

It would be better if these could be handled by users in simpler API.

Originally posted by @jsdw in #962 (comment)

[niklasad1]

In essence, we could send a custom close message when this fails or terminated but is this is heavily depending on the use-case and that's the reason jsonrpsee doesn't send out this message. It's possible to manually send out on ordinary notification or close by the APIs but easy to shot oneself in the foot and return early from the closure.

As we have discussed it could be possible with these options:

1. Add a closure to register_subscription to provide a default close value and/or use the close reason as error (this is tricky because it would need to be added to the proc macro API as well)
2. Refactor the subscription callback to return Option<Result<SubscriptionMessage, SubscriptionMessage>> (None: no notification is sent, Some(Ok(m)): a notification with the result field is sent, Some(Err(e)): a notification with the error field is sent

Option is 2) is simple to implement but it's reduces the ergonomics.

	let mut module = RpcModule::new(());
	module
		.register_subscription("sub", "_", "unsub", |_, pending, _| async move {
			// this just return `None` i.e, no extra notification is sent
                        // the connection is most likely closed
			let sink = pending.accept().await.ok()?;
                        
                        // if the JSON value couldn't be serialized sent out an error notification
		        let msg = SubscriptionMessage::from_json(&json).map_err(|e| Some(SubscriptionMessage::from_str(&e.to_string()))?;
			 
                        // if send fails here the connection is closed, no need to send an notification
                        // return None here 
                        sink.send(msg).await.ok()?;
	
                        // Send a regular subscription notification closed 
			Some(Ok(SubscriptionMessage::from_str("Closed")))
		})
		.unwrap();

[niklasad1]

//cc @lexnv @jsdw what do you guys think about Option 2)?

It's a bit awkward but it's the best I could come up with.

[niklasad1]

Closed by #1034