[D8] Allow forms to set custom validation error messages on required fields

This is the respective issue for https://www.drupal.org/project/drupal/issues/742344, a blocker for the #5346 backport, and related to #1040.

Problem/Motivation

Accessibility guidelines recommend specific, meaningful error messages when required form elements are empty. However, FAPI currently does not support custom error messages for required elements.

Proposed resolution

Support a #required_error property for form elements. If set, this message will be used when a required form element is empty, instead of the default "Field x is required." Additionally set a #required_but_empty flag automatically during validation when the required field is empty. This is an internal property and is not set manually. It allows #element_validate handlers to set a custom required error message, but without having to re-implement the complex logic to figure out whether the field value is empty.

Remaining tasks

Commit feature and document these properties. The feature is a small API improvement that is applicable to both D8 and D7.

User interface changes

None.

API changes

Minor, backwards-compatible addition only. No existing code will be affected. Addition of two FAPI element properties:

• #required_error
• #required_but_empty

Advocate for this issue: @klonos