[FIX] google_gmail, microsoft_outlook : move UserError to ir_mail_server

The orginal PR and it's forward ports https://github.com/odoo/odoo/pull/121048


intorduced an unexpected AttributeError when using OAuth for incoming
mail servers (fetchmail.server). Since `smtp_user` is not a defined field
in fetchmail.server (it uses the field `user` instead), we had to change
the approach.

To prevent this error, we move the UserError call into the respective
ir_mail_server models, which should check the contrains at that level.
This means that before the form gets saved, trying to connect using an
OAuth account, should prompt the user to first specify an smtp_user before
proceeding.

closes odoo/odoo#122222

Signed-off-by: Stéphane Debauche (std) <std@odoo.com>