[FIX] account: better error message when regenarating opening entries (opw 606923)

If we try to generate twice entries on the same fiscal year, we can get completly unrelated errors ("You can only reconcile journal items with the same partner").
With this, we make sure people will first cancel the entries before regeneraing the entries.
[FIX] sale: only propose services for pay advances

When creating an invoice from a sale order, if the user choose to invoice a fixed price (deposit), he has the possibility to choose an advance product. As described in the help message, this product should be a service product. Therefore, we add a domain so only services are displayed in the dropdown list