[FIX] partner_autocomplete: update log when sending partner to sync fails

In the start_sync method, the logger is updated to use the 'warning' level
instead of the 'error' level. This change reflects a less severe logging level
for cases where sending a partner to sync fails.

The changes have been made to reduce the noise level in the sentry.

sentry-3955309841

closes odoo/odoo#126055

X-original-commit: c1b608c2
Signed-off-by: Louis Baudoux (lba) <lba@odoo.com>