Fixed 404 on courseware when users last visited subsection becomes unpublished.

"Page not found" if student visits courseware after the section they visited becomes unpublished.
LMS-6492
[Never use time.sleep()](https://github.com/edx/bok-choy/blob/master/docs/guidelines.rst#never-use-timesleep) But if one have no choice other than using time.sleep() then you can use it.