Use standard required error message

Problem/Motivation

I want the required error message from the Conditional Fields module to be consistent with the rest of the framework.

This module can conditionally set a field as required. This is not validated in the browser via an HTML attribute but also in the backend.

Currently the required error message is '%name is required.' introduced in #2833368: Fix conditionally required fields message and label. The relevant code is src/ConditionalFieldsFormHelper.php:623.

Drupal core uses '@name field is required.' as default required error message. This can be seen in core/lib/Drupal/Core/Form/FormValidator.php:299.

The message from Drupal core is also used at multiple places.

Steps to reproduce

1. Configure an node type with
   • required field field_dependee.
   • optional field field_dependent.
2. Configure a field condition so that field_dependent becomes required when field_dependee has no value.
3. Open the creation form page for the node type.
4. Disable frontend form validation (https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/form#novalidate).
5. Submit the form without filling any field.
6. Expect the required error message for field_dependee and field_dependent to be identical

Proposed resolution

Use the Drupal required error message.

Remaining tasks

• Implement test.
• Implement solution.
• Fix test case regarding custom required error message.

User interface changes

The required error messages are consistent.

API changes

None.

Data model changes

None.