Validate no longer raises warnings
I marked this as bug because I think this was an unexpected consequence of #9796 (closed).
Since then, _validate() is decorated with without_rule():
https://hg.tryton.org/trytond/file/tip/trytond/model/modelstorage.py#l1060
which uses set_user(0), which in turn disables warnings:
https://hg.tryton.org/trytond/file/tip/trytond/res/user.py#l1019
This means that raising a UserWarning in validate() does not have the expected behaviour which is showing the message to the user.