to facilitate expression error handling

📚 Use ©

Used by other modules; these exports got removed when I replaced an ``import *`` earlier.

In the process:
* Update based on latest xcg/templates/odoo_module.
* Fix existing MailTemplate converter (found by these new tests).

Fix issue with Switch and the use of Skip as a converter, also make switch send Skip when nothing matches its rules.

Fix issue with RelationToMany that always convert to Skip when send_empty is False.

Fix issue with chaining relation converter with empty values.

update to use latest template, using | instead of Union as allowed by python 3.10 as it is more readable