[IMP] account_usability_akretion : Replace error by warning in case a user try to reverse an already reversed account move

The goal is to allow multiple refund for a single invoice. It may happen to refund invoice partially and they refund again partially later.

account_usability_akretion/__manifest__.py

@@ -30,6 +30,7 @@
         'wizard/account_invoice_mark_sent_view.xml',
         'wizard/account_group_generate_view.xml',
         'wizard/account_payment_register_views.xml',
+        'wizard/account_move_reversal.xml',
         'security/ir.model.access.csv',
 #        'report/invoice_report.xml',  # TODO
         "views/res_partner.xml",