filestore not None.  mitxhome.html now properly automatically lists
all courses given in settings.COURSE_SETTINGS.  mitxhome now served
from multicourse.views.  optioninput.html was missing; fixed. cleaned
up access to request.session for coursename in courseware/views.py

Display specific error messages when parts of the courseware break, rather than returning a 500 indicating that the whole site is down

This problem was introduced in 232e758c
and caused the check answer button to not functioning.

This fixes the bug in answers reset feature

Re-raise an integrity error during user creation if it's not caused by something we know to check for

Catch user integrity errors, and then try and figure out what caused them (which removes a race condition)

Made sure that histogram graph doesn't show up for modules without scores. Modified look of histogram graph.