[Wordpress] Make featured image required

function.php

add_action('save_post', 'custom_check_thumbnail');
add_action('admin_notices', 'custom_thumbnail_error');
function custom_check_thumbnail($post_id) {

  // change to any custom post type
  if(get_post_type($post_id) != 'post')
  return;

  if ( !has_post_thumbnail( $post_id ) ) {
    // set a transient to show the users an admin message
    set_transient( "has_post_thumbnail", "no" );
    // unhook this function so it doesn't loop infinitely
    remove_action('save_post', 'custom_check_thumbnail');
    // update the post set it to draft
    wp_update_post(array('ID' => $post_id, 'post_status' => 'draft'));
    add_action('save_post', 'custom_check_thumbnail');
  } else {
    delete_transient( "has_post_thumbnail" );
  }
}

function custom_thumbnail_error() {
  // check if the transient is set, and display the error message
  if ( get_transient( "has_post_thumbnail" ) == "no" ) {
    echo "
      <div id="message" class="error">
        You must select Featured Image. Your Post is saved but it can not be published.
      </div>";
    delete_transient( "has_post_thumbnail" );
  }
}