Adds a course for the user after conducting a set of sanity checks.
            Return the title of the course or an exception.

def add_course_safe(self, id):
        '''
            Adds a course for the user after conducting a set of sanity checks.
            Return the title of the course or an exception.
        '''
        course = get_object_or_404(Course, pk=int(id), active=True)
        if course not in self.courses.all():
            self.courses.add(course)
        return course.title