[FIX] account: autopost draft entries

The following commit https://github.com/odoo/odoo/commit/0015662d166f5e07bcbd6c5e9e14f8837ffa58a3

 was incorrectly done because the records are already limited to a 100 in the search query. We therefore remove redundant loop.

We also moved up all possible UserError in the _post() method before the soft computation such that if any UserError is raised (with the autopost cron for example), the soft computation will not be executed.

task-3482939

closes odoo/odoo#133485

Signed-off-by: William André (wan) <wan@odoo.com>