QmlDesigner: Show message box if StatesList.qml cannot be created

I also removed the hard assert.

Change-Id: Ia15d9f82fb63e7d8dd6acf8547b9e96bad838d3f
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>