[FIX] point_of_sale: throw error if incorrect configuration

- Install a new DB without demo data, with only the `point_of_sale`
  module
- Do not configure the accounting
- Open the POS, sell a product

At payment, an error is thrown at `this.cashregister.journal_id[1]`
because `this.cashregister` is `undefined`.

We throw a proper error instead.

opw-1891901

closes odoo/odoo#27683