Comment 7 for bug 1795563

Here's the latest discussion in the forum: https://forum.kicad.info/t/problem-creating-fill-zones-and-board-outline/13984

A very quick solution without making drastic changes to code or changing existing would be to make DRC catch it. Right now "graphic polygons" seem to be handled like segments. Polygons aren't handled at all. DRC could just take existence of both as an error and give a corresponding warning (not just "Board outline does not form a closed polygon").