Log client disconnected on ring sse

This is only detected when ring sse attempts to send on the closed
channel. It is not detected when the client actually
disconnects. Lower layers in the stack do notice the disconnect and
drop the sse stream so pings will stop being sent, but we don't have
any drop handler on the ring substream so we don't notice until we try
to use it.

src/main.rs

@@ -17,7 +17,7 @@ fn button_pressed(_level: Level, clients: &Arc<Mutex<SmallVec<[mpsc::Sender<()>;
     match tx.try_send(()) {
       Ok(_) => true,
       Err(mpsc::error::TrySendError::Full(_))   => true, // we just get some free debouncing
-      Err(mpsc::error::TrySendError::Closed(_)) => false
+      Err(mpsc::error::TrySendError::Closed(_)) => { info!("Event client disconnected"); false }
     }
   });
 }