'Return the CourseDescriptor of the course related to this CCX'

@lazy
def course(self):

    store = modulestore()
    with store.bulk_operations(self.course_id):
        course = store.get_course(self.course_id)
        if ((not course) or isinstance(course, ErrorDescriptor)):
            log.error(u'CCX    {0}    from    {2}    course    {1}'.format(self.display_name, self.course_id, (u'broken' if course else u'non-existent')))
        return course
