Raise osv.except_osv has no effect in a popup

In a wizard popup (new window with 'type': 'ir.actions.act_window' and 'target': 'new'):

If you have a object type button raising an except_osv, you will get an error popup, unless you first press a button returning True.
In the latter case, nothing happens, which quite uninformative and puzzling for the user.

Please install error_popup_wizard module provided to reproduce: it adds a popup button on res.user form view.

This pop up has 2 buttons: True and Error.

If you click on Error directly, you get the error popup.
If you first click on True and then Error, nothing happens.