Add overflow behavior to pillbox_used and pillbox_large_used

[tmqCypher]

Summary

None

Purpose of change

Fixes overflowing pillboxes (mentioned in #72594)

Describe the solution

Set the overflow behavior of pillbox_used and pillbox_large_used to "discard" to keep them from raising errors.

Describe alternatives you've considered

Modifying pillbox_contents to not overflow pillboxes. The total possible volume of pillbox_contents isn't greater than the total capacity of a pillbox, so I suspect that the overflows are being caused by the contents not being ideally packed into each pocket, which I had not accounted for (for example, 2 10ml pockets can hold 2 1ml pills and 3 5ml pills, but only when both 1ml pills are placed in the same pocket). While it would be ideal to have a pillbox_contents who's least efficient possible packing doesn't overflow pillboxes in the first place, just adding an overflow behavior makes this much easier to maintain and much less likely to cause errors in the future.

Testing

Tested in-game using the itemgroup debug tool.

Additional context