DAL: Rework the interface of dal_pages_request.mli + Revisit ml for better errors handling

The interface is quite outdated:

• It references {!Store.Dal_slot_pages}, which doesn't exist anymore
• Error Dal_slot_not_found_in_store is not used anymore

Also ml file might need improvements for better errors handling

Also, maybe add a test where the requested page is not available yet (emulate a lagging or offline DAL node)