[IMP] payment: Improved error message for an empty field in payment.acquirer form view

- payment.acquirer have many fields with 'required_if_provider'
   attribute and while saving form it gave an error like "Required
   fields not filled", so it was confusing to know  which required fields were empty.

 - after this commit, it is displayed like "Required fields not filled (field name)"

opw:1881054

closes odoo/odoo#26833

addons/payment/models/payment_acquirer.py

@@ -196,9 +196,13 @@ class PaymentAcquirer(models.Model):
     def _check_required_if_provider(self):
         """ If the field has 'required_if_provider="<provider>"' attribute, then it
         required if record.provider is <provider>. """
+        empty_field = []
         for acquirer in self:
-            if any(getattr(f, 'required_if_provider', None) == acquirer.provider and not acquirer[k] for k, f in self._fields.items()):
-                return False
+            for k, f in acquirer._fields.items():
+                if getattr(f, 'required_if_provider', None) == acquirer.provider and not acquirer[k]:
+                    empty_field.append(self.env['ir.model.fields'].search([('name', '=', k), ('model', '=', acquirer._name)]).field_description)
+        if empty_field:
+            raise ValidationError((', ').join(empty_field))
         return True
 
     _constraints = [