[FIX] website_slides: fix archived slides traceback

before this commit:
When a user tries to see an archived slide, it throws an internal server error
instead of 404 page

after this commit:
404 page will be shown if a slide is archived

task- 2018147

closes odoo/odoo#34198

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