Make outline print scene graph error
This is to prevent the outline potentially crashing kwin when a graphics reset occurs.
If the QSGRenderLoop fails to create an OpenGL context (can happen during graphics reset), it will check whether the QQuickWindow::sceneGraphError signal is connected. If it is, the QSGRenderLoop will emit the signal, clean up some things and continue execution. If it is not, the QSGRenderLoop will crash the app.
Edited by Vlad Zahorodnii