This snippet must return a value which can be of different types. The only way to indicate
successfull validation is to return a boolean true value.
To indicate validation error, a number of result types are possible:
A boolean false: the validator must then have
set a validation error on the validated widget or one of its children.
A ValidationError: this error is then set on the validated widget.
A String: a validation error using that string as a non-i18nized message is
then set on the validated widget
An XMLizable such as I18nMessage: this
xmlizable is used to build a validation error that is set on the validated widget.
The JavaScript snippet has the "this" and "widget" variables set to the validated widget, and, if the form
is used in a flowscript, can use the flow's global values and fonctions and the cocoon object.