[FIX] point_of_sale: offline invoicing => backend printing

In a pos session:
OFFLINE
make an order with invoicing , try to validate
    The order stays there because it needs to be validated by the server
make another non invoiced order, validate

ONLINE
make another order
    At validation, all orders will be pushed to the server

Before this commit, when trying to validate the invoiced order
the report download couldn't find the order id, and crashed
This was because the order in question was already pushed
but treated as a non invoiced order

After this commit, an "warning" message is displayed to the customer
saying he/she has to print the invoice from the backend.
In most cases it is enough and acceptable, since a customer would actually leave the premises
and come back later for the invoice
It is also safer in terms of data consistency to keep pushing all orders once the connection is back

OPW 1918044

closes odoo/odoo#30485

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