Fix: End tick_loop() when the receiver is gone.

Currently, `tick_loop()` would keep printing the trace message every
tick even when the receiver (Raft main loop) is gone in this form:

`INFO openraft::core::tick: .../tick.rs:70: Tick fails to send, receiving end quit: channel closed`

If the tick message fails to send, then terminate the loop, since every
future message will fail to send as well.

Also adjust the trace message to better describe what happened.

openraft/src/core/tick.rs

@@ -66,8 +66,9 @@ where C: RaftTypeConfig
             }
 
             let send_res = self.tx.send(Notify::Tick { i });
-            if let Err(e) = send_res {
-                tracing::info!("Tick fails to send, receiving end quit: {e}");
+            if let Err(_e) = send_res {
+                tracing::info!("Stopping tick_loop(), main loop terminated");
+                break;
             } else {
                 tracing::debug!("Tick sent: {}", i)
             }