When trying to add a QR code to an invoice, if some conditions weren't met, the user would more often than not get a message simply stating :
"The chosen QR code is not eligible with this invoice".
This was confusing to the user, since the reason for a QR code to not be eligible are multiple : invalid IBAN, unavailable in X country, wrong currency...
Modified the _eligible_for_qr_code function to _get_error_messages_for_qr() so it returns an error message if the qr code is not eligible, and None otherwise.
task-3069753
closesodoo/odoo#116482
Signed-off-by: Olivier Colson (oco) <oco@odoo.com>