Filters the errors encountered when a new user is being registered.

The filtered WP_Error object may, for example, contain errors for an invalid
or existing username or email address. A WP_Error object should always be returned,
but may or may not contain errors.

If any errors are present in $errors, this will abort the user’s registration.

$errors = apply_filters( 'registration_errors', $errors, $sanitized_user_login, $user_email );