Lesson on common python error messages

For teaching round 8.5, I was initially going to a lesson on `git annex`. However, after a discussion at PyCon about teaching error messages, @gvwilson suggested that I do a lesson on error messages instead, so here it is!

One thing I was a little unsure about was notebook vs. webpage -- the challenges in this lesson require being able to modify and execute code, but obviously this is not something you can do when it is converted to HTML. Is this ok? Or should I redo the challenges so they are more appropriate for a static webpage?