Dexterity modal fix

This fixes a problem when submitting a Dexterity add form (from a modal) which has a required field in a fieldset other than the first fieldset. In this case Dexterity incorrectly returns a portal message of type INFO (_"Info: there were some errors"_), misleading the modal pattern into thinking that there were no form errors.

I also added an option to specify the selector for the form fields errors and updated the modal pattern's docs.