displaying an error before update_post_meta

Short of doing some jQuery validation, I think the only option is wp_die().

add_action( 'save_post', array( $this, 'save_custom_items_data' ), 10, 2 );

public function save_custom_items_data( $post_id, $post ) 
{
    if( count( $related_items ) > 5 ) 
    {
        wp_die(
            'Error, 5 items max.', 
            'Error',  
            array( 
                'response' => 500, 
                'back_link' => true 
            )
        );    
    } 
    else 
        update_post_meta( $post_id, '_custom-meta-items', $related_items );
}

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