[FIX] mail_plugin: fix access error when enriching some partners

Bug
===
When we enrich a partner we don't have access to, it will try to read
it, but because of the ACL if will crash. To fix that issue, we don't
return the partner if we can't access it and we show an error message
in the UI instead of letting it crash.

Task-3344536

closes odoo/odoo#134767

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>