fix: simplify abort handling and add missing emits

Signed-off-by: flow <flowlnlnln@gmail.com>

launcher/InstanceImportTask.h

@@ -80,6 +80,7 @@ private slots:
     void downloadSucceeded();
     void downloadFailed(QString reason);
     void downloadProgressChanged(qint64 current, qint64 total);
+    void downloadAborted();
     void extractFinished();
     void extractAborted();