[IMP] web_unsplash : update unsplash messages

The goal of this commit is to update all unsplash alert messages :

- message : "unsplash key not found"
- message : "search temporarily unavailable"
- message : "unauthorized key"
- message : "something went wrong"

and replace them with cleaner content.

task-2341558

closes odoo/odoo#59014

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>