Using Redirect Args to Display Errors

add-redirect-args.php

if ( ! empty( $errors ) ) {
    add_filter('redirect_post_location', function( $location ) use ( $errors ) {
        return add_query_arg( 'gistpen-errors', implode( ",", $errors ), $location );
    });
}

display-get-errors.php

if ( array_key_exists( 'gistpen-errors', $_GET) ) { ?>
    <div class="error">
        <p><?php
            echo 'The post saved with error codes: ' . $_GET['gistpen-errors'];
        ?></p>
    </div><?php
}