Tell the user if the SWE data files are not found

[gergelypolonkai]

If the SWE data files are not found, major planets can still be calculated (well, roughly, which is enough for a chart), but asteroids can’t. This will render the chart somewhat unusable for these asteroids, as they will appear at 0°.

Final solution would be a message box, which should list the planets that can’t be calculated, while hiding such planets on the chart.