Explicitly cover unexpected disconnects during login

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/LoginSessionHandler.java

@@ -20,6 +20,7 @@ import net.kyori.text.TextComponent;
 import javax.crypto.Mac;
 import javax.crypto.SecretKey;
 import javax.crypto.spec.SecretKeySpec;
+import java.io.IOException;
 import java.security.InvalidKeyException;
 import java.security.NoSuchAlgorithmException;
 import java.util.concurrent.CompletableFuture;
@@ -100,6 +101,15 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
         }
     }
 
+    @Override
+    public void disconnected() {
+        CompletableFuture<ConnectionRequestBuilder.Result> future = connection.getMinecraftConnection().getChannel()
+                .attr(VelocityServerConnection.CONNECTION_NOTIFIER).getAndSet(null);
+        if (future != null) {
+            future.completeExceptionally(new IOException("Unexpectedly disconnected from remote server"));
+        }
+    }
+
     private void doNotify(ConnectionRequestBuilder.Result result) {
         CompletableFuture<ConnectionRequestBuilder.Result> future = connection.getMinecraftConnection().getChannel()
                 .attr(VelocityServerConnection.CONNECTION_NOTIFIER).getAndSet(null);