Efficiently fail to place chess board.

*If it fails to place the chess board:*

    **Before**, using `isFree = false`, when it printed "Cannot place chess board", it still had to make its way out of the three `for` loops and past the whole `if (isFree) then` statement all the way to the end of `after_place_node`.

    **Now**, using `return`, as soon as it prints "Cannot place chess board" it immediately exits `after_place_node`.

*If it succeeds in placing the chess board:*

    Not as much of a difference; it won't have to think about the lines `local isFree = true` and `if (isFree) then` 


(Somehow I referenced the wrong pull request or issue, and the other fix for this got closed with an unrelated issue/request; so I deleted my branch, rebranched from master, recommitted, and re-requested.)