greeter: Fix signal invocation on QNetworkReply
The error signal is errorOccurred now. While at it, use compile-time checked invokeMethod and make use of qArg (no need for Q_ARG anymore).
Fixes
QMetaObject::invokeMethod: No such method ScreenLocker::AccessDeniedNetworkReply::error(QNetworkReply::NetworkError)
QMetaObject::invokeMethod: No such method ScreenLocker::AccessDeniedNetworkReply::error(QNetworkReply::NetworkError)