Subscription Sink improve ergonomics of send API

[lexnv]

The send API of the subscription sink can return the following:

• Ok(true): message was sent
• Ok(false): message not sent, because the client is disconnected
• Err(err): Error regarding serialization failure

jsonrpsee/core/src/server/rpc_module.rs

Lines 888 to 902 in dbac4bd

	/// Send a message back to subscribers.
	///
	/// Returns
	/// - `Ok(true)` if the message could be send.
	/// - `Ok(false)` if the sink was closed (either because the subscription was closed or the connection was terminated),
	/// or the subscription could not be accepted.
	/// - `Err(err)` if the message could not be serialized.
	pub fn send<T: Serialize>(&mut self, result: &T) -> Result<bool, serde_json::Error> {
	// Cannot accept the subscription.
	if let Err(SubscriptionAcceptRejectError::RemotePeerAborted) = self.accept() {
	return Ok(false);
	}
	self.send_without_accept(result)
	}

This is a bit tricky to work with, given that users might try to match
only errors (ie if let Err(err) = sink.send() { stop_service(); }).

To improve the API, ensure that on success Ok(()) is returned and on
failure have a dedicated error to differentiate between disconnect errors
or serde errors:

pub enum SubscriptionSinkError {
  ClientDisconnected,
  Serialization(serde::Error),
}

[niklasad1]

Ideally, folks shouldn't don't have to bother with this API at all and use pipe_from_sink but there might some edge-cases where one want to use send.

I think we designed it just like that around the pipe_from_sink i.e, not to regard ConnectionClosed as an error.
In addition serde::Serialization errors are most likely bugs and should not really occur.

Anyway, I'm all good with regarding all send errors as an error instead and to make this API cleaner.

We could also have some specific error type such as https://docs.rs/futures/latest/futures/channel/mpsc/struct.TrySendError.html#

[niklasad1]

Closed by #962