Ensure that the unit price on the invoice line is not negative for Peppol

Closes #14504

modules/edocument_peppol/exceptions.py

@@ -1,12 +1,16 @@
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
 
-from trytond.exceptions import UserWarning
+from trytond.exceptions import UserError, UserWarning
 
 
 class InvoicePeppolRequired(UserWarning):
     pass
 
 
+class InvoicePeppolError(UserError):
+    pass
+
+
 class PeppolServiceError(Exception):
     pass