[FIX] google_calendar: Change logger type from error to exception

In sentry we receive like 2k syncing issue from google_calendar.

This commit converts `_logger.error` to `_logger.warning` because we don't
need an erro for that

sentry-4121123364
https://online.sentry.io/issues/4121123364



closes odoo/odoo#119616

Signed-off-by: Arnaud Joset <arj@odoo.com>