[FIX] base: invalid UNIQUE constraint just warn

When a submodule overrides an old UNIQUE constraint with a new one,
records in that module may respect the new one and not the old one.
As the old module is updated, it would fail giving an ERROR message,
and therefore blocking the Odoo.sh deployment pipeline.

i.e. website_sale (old): res_users_login_key -> ['login', 'website_id']
             base (new): res_users_login_key -> ['login']

With this patch, the error level is changed from ERROR to WARNING,
leaving Odoo.sh free to continue the build deployment, as the error
was not a blocking one.

closes odoo/odoo#79042

Signed-off-by: Olivier Dony (odo) <odo@openerp.com>