Function is_out_of_tries 

pub async fn is_out_of_tries(
    conn: &mut PgConnection,
    user_id: Uuid,
    exercise: &Exercise,
    course_or_exam_id: CourseOrExamId,
) -> ModelResult<bool>

Whether the user has used up the try limit on every slide of the exercise, so no further submission is accepted. Always false for an exercise without a try limit or without slides; a slide nobody has submitted to has all its tries left.