Fix textual typos
Replace filesystem-based class verification with file_exists()

Switch to a try/catch model, too.

Further to
https://github.com/statedecoded/statedecoded/commit/06bfbb54af65f697414f
64b2d0687a1a357263f1#commitcomment-4417423

@krues8dr, I've written this faster than I should, as I'm about to have
a very busy afternoon and then be gone for days. I tested it and it
worked fine, but would you look over it and make sure I haven't done
anything dumb here? I'm worried about the `== FALSE` bit. I might have
inverted some logic.