Function verify_uploads_belong_to_exercise 

pub async fn verify_uploads_belong_to_exercise(
    conn: &mut PgConnection,
    exercise_id: Uuid,
    user_id: Uuid,
    requested: &[Uuid],
) -> Result<(), ControllerError>

Checks every id a submit names against the uploads recorded for this exercise and user.

Unlocked, so a caller that goes on to record the association must follow this with lock_and_verify_uploads_are_usable inside that transaction.