catch `fs.statSync()` error in `loadExercise()`

Hello, @rvagg and other nice `workshopper` people!

I've started looking at `workshopper` to add i18n support and I stumbled across a small problem: the `fs.statSync()` function throws an error rather than return a falsy value if the exercise file doesn't exist. This pull request adds a `try...catch` to catch the error.

I've got another general patch for the bundled example and `makews.js`. Would you rather I send a separate pull request for that patch or just add it to this one?

Thanks a bunch for `workshopper`!