Handle non-numeric watcher sliders the same way Scratch does

[adroitwhiz]

I haven't looked too much into exactly what Scratch does to a watcher slider when the watcher's value isn't a number, but it's more complex than just "cast to a number".

It appears that non-numeric values set the slider to its middle value (upper-middle if there are an odd number of values). Not all values that can be cast to numbers are--hexadecimal numbers, for instance, are also treated as non-numeric.