[ws server]: unsubscribe_call to non-active subscription should indicate error

[niklasad1]

		// Subscribe call.
		let sub_resp = rpc
			.call_with("subscribeSomething", Vec::<()>::new())
			.await
			.unwrap();
		let sub_id: SubscriptionId = deser_call(sub_resp);

		// Unsubscribe
		assert_eq!(
			rpc.call_with("unsubscribeSomething", [sub_id.clone()]).await,
			Some(r#"{"jsonrpc":"2.0","result":"Unsubscribed","id":0}"#.into())
		);

		// Should indicate that subscription was already closed and not active anymore!
               // but doesn't
		assert_eq!(
			rpc.call_with("unsubscribeSomething", [sub_id]).await,
			Some("{\"jsonrpc\":\"2.0\",\"error\":{\"code\":-32602,\"message\":\"Invalid subscription id.\"},\"id\":1}".into()),
		);

[niklasad1]

Closed by #561