Some improvements to novice Python function lesson

Reorganized things a little bit; instead of showing a good `fahr_to_kelvin` first, we start with a bad one and do basic debugging to figure out why it's not working and fix it. This adds an extra learning objective: basic debugging.

There's an argument to be made that the debugging section makes this lesson too long, and perhaps this isn't the best place to introduce debugging, so opinions on that would be very welcome. Thanks!