Example of throwing errors during post creation

gistfile1.txt

add_action('save_post', 'wdb_check_thumbnail');
add_action('admin_notices', 'wdb_thumbnail_error');

function wdb_check_thumbnail($post_id) {

    // convert to a custom post type 
    if(get_post_type($post_id) != 'post')
        return;
    
    if ( !has_post_thumbnail( $post_id ) ) {
        // Show the users an admin message
        set_transient( "has_post_thumbnail", "no" );
        remove_action('save_post', 'wdb_check_thumbnail');
        // make the post a draft
        wp_update_post(array('ID' => $post_id, 'post_status' => 'draft'));

        add_action('save_post', 'wdb_check_thumbnail');
    } else {
        delete_transient( "has_post_thumbnail" );
    }
}

function wdb_thumbnail_error()
{
    if ( get_transient( "has_post_thumbnail" ) == "no" ) {
        echo "<div id='message' class='error'><p>You must select featured image in order for your post to be published.</p></div>";
        delete_transient( "has_post_thumbnail" );
    }

}