RegularStarBox: Error message if box won't work

For very small boxes (relative to the thickness) with many corners there
may not be space to make the two halves fit together. Issue an
ValueError that is shown in the UI.

Thanks to DillonSimeone <https://github.com/DillonSimeone> for reporting
this!

Resolves: #671

boxes/generators/regularstarbox.py

@@ -35,7 +35,9 @@ def __call__(self, length, **kw):
 
         if li > 0:
             poly = [lo-1, (90, 1), d+t-1, -90, ls1+ls2, -90, d-t, (90, t)]
-        self.polyline(*(poly + [li-2*t] + list(reversed(poly))))
+            self.polyline(*(poly + [li-2*t] + list(reversed(poly))))
+        else:
+            raise ValueError("Box is too small and has too many corners to work properly")
 
     def startwidth(self) -> float:
         return self.settings.thickness