'Unenroll a user from a course given their email. This saves immediately.
User lookup errors are logged but will not throw an exception.
`email` Email address of the User to unenroll from the course.
`course_id` is our usual course_id string (e.g. "edX/Test101/2013_Fall)'

@classmethod
def unenroll_by_email(cls, email, course_id):

    try:
        user = User.objects.get(email=email)
        return cls.unenroll(user, course_id)
    except User.DoesNotExist:
        log.error(u'Tried    to    unenroll    email    %s    from    course    %s,    but    user    not    found', email, course_id)
