Forces a featured image to set and displays error if one isn't set.

functions.php

	add_action('save_post', 'pk_require_post_thumbnail');
	add_action('admin_notices', 'pk_post_thumbnail_error');

	function pk_require_post_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', 'pk_require_post_thumbnail');
	        
		// update the post set it to draft
	        wp_update_post(array('ID' => $post_id, 'post_status' => 'draft'));
	        add_action('save_post', 'pk_require_post_thumbnail');

	    } else {
	        delete_transient( "has_post_thumbnail" );
	    }
	}

	function pk_post_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'><p><strong>You must select Featured Image. Your Post is saved but it can not be published.</strong></p></div>";
	        delete_transient( "has_post_thumbnail" );
	    }
	}