[FIX] mail: access link generaTION

Catch only the error related to the access of the linked record.
Previous was hidding error to access the ir.config_parameter and eventual other
errors from the orm.
Uses SUPERUSER_ID to access instance URL (e.g. portal has no access to it).
Fixes #4234 #4234
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-linux64