[rpc module]: return error from subscriptions to make error handling more ergonomic

[niklasad1]

Recently the subscription code got refactor where one must reject or accept a subscription manually and because
SubscriptionClosed is not treated as an error the closures must not return Result.

However, it's quite awkward to so:

Example:

	module
		.register_subscription("sub_one_param", "sub_one_param", "unsub_one_param", |params, pending, _| {
		       // not possible to return Result here.
                       let idx = match params.one::<usize>() {
				Ok(idx) => (idx, sink),
				_ => {
                                    pending.reject("some special error);
                                    return;
                               }
			};
		       
                      let sink = match pending.accept() {
                         Some(sink) => sink,
                         None => return;
                      };

                      sink.pipe_from_stream(...);
		})
		.unwrap();

So it would be more ergonomic / less awkward if it would be possible to do:

	module
		.register_subscription("sub_one_param", "sub_one_param", "unsub_one_param", |params, pending, _| {
		       // pending implement drop and will reject the subscription with default error
                       let idx = params.one()?;
                       let sink = pending.accept()?;
		       
                        sink.pipe_from_stream(...);
                        Ok(())
		})
		.unwrap();

The tricky part is that if one wants to reject a call explicitly then the error type has to be cloned or borrowed.
So that's rationale why the API is like is at the moment.