[DAL]: Move all errors related to Dal to dal_errors_repr.ml

The errors Sc_rollup_dal_slot_already_registered and Sc_rollup_requested_dal_slot_subscription_of_future_level are currently part of sc_rollup_errors.ml, but they should actually be in dal_errors_repr.ml`

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information