'Remove the user from a given course. If the relevant `CourseEnrollment`
object doesn\'t exist, we log an error but don\'t throw an exception.
`user` is a Django User object. If it hasn\'t been saved yet (no `.id`
attribute), this method will automatically save it before
adding an enrollment for it.
`course_id` is our usual course_id string (e.g. "edX/Test101/2013_Fall)
`skip_refund` can be set to True to avoid the refund process.'

@classmethod
def unenroll(cls, user, course_id, skip_refund=False):

    try:
        record = cls.objects.get(user=user, course_id=course_id)
        record.update_enrollment(is_active=False, skip_refund=skip_refund)
    except cls.DoesNotExist:
        log.error(u'Tried    to    unenroll    student    %s    from    %s    but    they    were    not    enrolled', user, course_id)
