Fix copy_quizzes to use `quiz_dir`

[cansavvy]

Purpose/implementation Section

What changes are being implemented in this Pull Request?

This is the sequel to #80 because I reviewed it too quickly and it didn't copy the quizzes. :sad:

But now it does. I changed it to take the quiz_dir argument that a user specifies in the bookdown_to_leanpub() main function so it should inherit whatever that value is. Also added a message for if that directory isn't found.

[cansavvy]

Now to see if the checks pass.