Add materials for python errors lesson
Add "case sensitive" and "traceback" to glossary
Finish lesson with item errors, key points
Add cell tags for challenges
Add markdown file for errors lesson
Add note about tabs/spaces
Use better example than numbers from 1-10, since Python allows -1
Add section on IOError
Merging Jess Hamrick's lesson on error messages
Merge pull request #616 from gvwilson/jhamrick-python-errors-lesson

Resolving conflicts in @jhamrick's lesson on Python error messages