Returns true if the form contains any invalid fields. No fields will be marked as invalid
as a result of calling this; to trigger marking of fields use {@link #isValid} instead.
function() {
        return !!this.getFields().findBy(function(field) {
            var preventMark = field.preventMark,
                isValid;
            field.preventMark = true;
            isValid = field.isValid();
            field.preventMark = preventMark;
            return !isValid;
        });
    }