If custom field has bad value, cease saving post and display error

Hi!

I have now this:

add_action('save_post','save_details');
function save_details(){
	global $post;

	if($post->post_type == "booking") {
		update_post_meta($post->ID,'numberSeats', "" . intval( $_POST['numberSeats'] ) );
	}
}

And I want it to be something like:

add_action('save_post','save_details');
function save_details(){
	global $post;

	if($post->post_type == "booking") {
                if( !check_if_good_value($_POST['numberSeats']) ){ /* DO NOT SAVE THE POST AND RETURN THE USER TO THE EDIT SCREEN WITH AN ERROR MESSAGE*/ }
		update_post_meta($post->ID,'numberSeats', "" . intval( $_POST['numberSeats'] ) );
	}
}

What sould I do to display an error message in this case instead of saving the post? Cann I display the already entered data as well?