Chapter 20: Consider error handling in final project

[betamos]

In Chapter 20, the final project, results are simply unwrapped. It wouldn't be too much extra to add error handling with ?, and it seems like a good exercise given that it's covered in the book. Can we add this? I can make a PR.

[steveklabnik]

This change would be too big to take a PR for; but it may be worth doing.