Using Redirect Args to Display Errors

add-redirect-args.php

if ($error) {
    add_filter('redirect_post_location', function( $location ) use ( $error ) {
        return add_query_arg( 'my-plugin-error', $error->get_error_message(), $location );
    });
}

display-get-errors.php

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