Respect exit codes when there are koans failing

This should help continuous integration systems like Travis-CI or Jenkins to determine whether the student has reached Zen illumination, automatically ;)

We are giving a python course at our organization:

https://github.com/pythonkurs

This pull request will help us monitor whether the students pass or fail the assignment (your koans).

@guillermo-carrasco, @vals, @OxanaSachenkova and @hugerth are senseis in this course.