Add some input validation to .make_ascii_maker

[jayqi]

There are some assumptions by .make_ascii_maker around the inputs. To be safe, it would be great to have some input validation there. If we want to maintain 100% code coverage, we'd also have to write tests.

For now this is an internal function so it's not a huge priority. It's a pain to validate (and then test the validation) for 3 different conditions on these 5 inputs. Maybe a good code-golfing exercise.