[FIX] google_gmail: fix google gmail oauth crashing

Bug
===
When an error occurs, the error received is wrongly stringified.

closes odoo/odoo#112290

X-original-commit: 1bf9278b
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>
Signed-off-by: Stéphane Debauche (std) <std@odoo.com>