Refactor ChangeTable scaling methods so that they create ValueErrors on bad input

[danielolsen]

Currently, if we try to scale a grid element based on a bad zone name or plant ID, all scaling for that grid element is removed, and an error message is printed. I think it would be more intuitive if a ValueError were raised, and any previous scaling were maintained. This would be consistent with the behavior when we add elements, and I believe that we discussed doing the same for scaling, but it fell off the priority list.