A mental model for error handling

[Damien Irving]

Comment from the reviewer (@ChristinaLK):

"I had the most difficulty in following chapter 10 (Error Handling); it was the one chapter where I felt like I didn't have a sufficient mental model to understand the details of the chapter. I'm not sure whether the solution is for me to have a better understanding of Python or for the chapter to more explicitly generate a mental model for errors and exceptions."

@joelostblom has offered to have a go at addressing this issue.

[Joel Ostblom]

I will PR this tomorrow, sorry for the delay.

[Christina K.]

@joelostblom if you tag me when you submit the PR, I will make a best effort to review. :)

[Greg Wilson]

I believe #550 fixes this.