Commit [1] introduced some code that relied on a cache we currently have for processed** images. In case of 404, that cache is not filled... and relying on it thus crashes.

This commit circles around the issue by taking profit of [2] which now forces `loadImage` to load a placeholder image in case of a 404. This makes sure that in case of a 404 during the initial call to `loadImage`, the rest of the stack works with the loaded placeholder image.

**Note: the current cache is not filled at the `loadImage` level, but later when processing the fetched image. This should be changed in the future (a task has been created).

See [3] and [4] which are example of unwanted 404 images due to other issues, which allowed to discover this bug to fix here.

Steps to reproduce:
- Go on the `website.s_picture_default_image` ir.attachment form view
- Change the URL value to a relative path that does not exist
- Go on a website page and edit
- Drop the "Picture" snippet => Crash => After this fix, no crash but you can also edit the image if you were
   able to click on it, it would use the placeholder image as source.

[1]: https://github.com/odoo/odoo/commit/567e5b58d544e4e56c3a68d148e862a516d77c4f
[2]: https://github.com/odoo/odoo/commit/0639c5028c69d8e29c8438b741cc4713d875b06d
[3]: https://github.com/odoo/odoo/pull/155015
[4]: https://github.com/odoo/design-themes/pull/767

Related to opw-3693055 and others (see [3] and [4]).

Forward-Port-Of: odoo/odoo#155275
Forward-Port-Of: odoo/odoo#155097