[FIX] mass_mailing: keep body of invalid saved mailing

There was an issue when a mailing save failed: the edited body of the mailing
was lost.

Impacted versions:

  - 15.0 and higher

Steps to reproduce:

  1. Create a mailing "Start from scratch" and add some elements
  2. Do not fill in the subject (or at least one required field)
  3. Save (it should fail because of the empty required field)

Current behavior:

  - The edited template is lost

Expected behavior:

  - Keep the edited template even if the save failed

This commit fixes the problem by properly awaiting the promise _doAction so that
the field's value will be set correctly.

The testing tour `mass_mailing_editor_tour` is updated to check that the
body_arch is kept after a failed save.

This issue was introduced in 15.0 with:
95754eb6

Task-2856742

closes odoo/odoo#91677

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>