Make sync method action more user friendly

The SQL constraint gives the user no idea how
to rectify the problem.

Instead, sync all the Contacts that can be, and
collect the ones that can't into a list to
display to the user in a ValidationError.

mass_mailing_list_dynamic/models/mail_mass_mailing_list.py

@@ -1,9 +1,11 @@
 # -*- coding: utf-8 -*-
 # Copyright 2017 Tecnativa - Jairo Llopis
 # License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl).
+from psycopg2 import IntegrityError
 
 from odoo import api, fields, models
-from odoo.tools import safe_eval
+from odoo.exceptions import ValidationError
+from ast import literal_eval
 
 
 class MassMailingList(models.Model):
@@ -42,9 +44,10 @@ class MassMailingList(models.Model):
         Partner = self.env["res.partner"]
         # Skip non-dynamic lists
         dynamic = self.filtered("dynamic")
+        errors = []
         for one in dynamic:
-            sync_domain = safe_eval(one.sync_domain) + [("email", "!=", False)]
-            desired_partners = Partner.search(sync_domain)
+            domain = literal_eval(one.sync_domain) + [("email", "!=", False)]
+            desired_partners = Partner.search(domain)
             # Remove undesired contacts when synchronization is full
             if one.sync_method == "full":
                 Contact.search([
@@ -55,11 +58,22 @@ class MassMailingList(models.Model):
             current_partners = current_contacts.mapped("partner_id")
             # Add new contacts
             for partner in desired_partners - current_partners:
+                try:
+                    with self.env.cr.savepoint():
                         Contact.create({
                             "list_id": one.id,
                             "partner_id": partner.id,
                         })
+                except IntegrityError:
+                    errors.append(
+                        'Cannot add Partner %s to "%s". '
+                        'Email %s already added.' %
+                        (partner.name, one.name, partner.email)
+                    )
+            if errors:
+                raise ValidationError('\n'.join(errors))
             one.is_synced = True
+
         # Invalidate cached contact count
         self.invalidate_cache(["contact_nbr"], dynamic.ids)