Discovery: Change abort code a bit

Now pressing pause in the UI will not show an error to the user.

src/libsync/syncengine.cpp

@@ -1194,6 +1194,7 @@ AccountPtr SyncEngine::account() const
 
 void SyncEngine::abort()
 {
+    qDebug() << Q_FUNC_INFO << _discoveryMainThread;
     // Aborts the discovery phase job
     if (_discoveryMainThread) {
         _discoveryMainThread->abort();