Remove dictionary references from 09-errors.md

[patrick-austin]

Based on the feedback on #809, rather than introducing the concept of a dict have instead modified the exercise to use a list and an IndexError. The solution to the exercise has been modified to reflect this, and have changed print_friday_message to print_sunday_message to make the mistake of providing 7 as the index more plausible.

The original issue also made points about moving the exercise earlier. Given this wasn't addressed in the PR comments I assume that this is fine.

closes #783