[configlet create] Give a warning if invalid practice exercise slug is passed in

[BNAndras]

Configlet create --practice-exercise <slug> should warn the user if the passed slug isn't present in problem-specifications. On the SQLite track, we just had a PR get merged where the author had a typo in the slug name from using configlet create. It'd be helpful if configlet create reminded users to update the name in the track's config.json. Currently, it's just the slug name, and I already made the mistake of not fixing the exercise title on a PR I made elsewhere.