Widget (FAPI-)validation messages do not display the field 'label' when the error is on a nth value (n > 1).

#271577 by pwolanin : Fix unsanitized text (optionwidgets, number, text, field labels, field descriptions)

- Fix jumbled multiple values when hitting the 'add-more' button with more than 10 values.
- Keep 'add more' button text consistent.

Fix mistake where undefined $module is used instead of $widget['module'] and add documentation of what the custom default value expects.