`grid.subgrid` defaults to the last index instead of an `IndexError`

[jon-wurtz]

The following code snippet, when run through an animator, generates the following animation. Observe that the Y destination index (7) is outside the bounds of the geometry (which goes from 0 to 4). This should raise an IndexError. Similarly, asking for index -1 gives you the second-to-last index instead of the last index.

@move
def foo_move():
    zone = spec.get_static_trap(zone_id="traps")
    src = grid.sub_grid(zone, [-1], [3])
    dst = grid.sub_grid(zone, [15], [7])
    waypoints = ilist.IList([src, dst])
    
    init.fill([spec.get_static_trap(zone_id="traps")])
    move_by_waypoints(waypoints, True, True)

error.mp4