Runtime crash with arc function and negative radius

[SiriusStarr]

If Canvas.arc is passed a negative value for its radius, it crashes with

Uncaught DOMException: CanvasRenderingContext2D.arc: Negative radius

This should probably at least be documented clearly in a warning, since one is usually reasonably insulated from runtime crashes with Elm. (Obviously, the use of a custom element here does away with that "guarantee," but it's still not entirely expected.)