feat(multiplayer): notify user if connection lost.

HMCLCore/src/main/java/org/jackhuang/hmcl/util/platform/ManagedProcess.java

@@ -119,6 +119,10 @@ public class ManagedProcess {
      */
     public void stop() {
         process.destroy();
+        destroyRelatedThreads();
+    }
+
+    public void destroyRelatedThreads() {
         relatedThreads.forEach(Thread::interrupt);
     }