How can show a message for products that are not in stock?

I am implementing custom stock management in Commerce. I followed the following steps.

1. I have added a stock integer field in product variant.

2. I have written a event subscriber which reduce this stock field value after purchasing.

   public static function getSubscribedEvents() {
     $events['commerce_order.place.post_transition'] = ['orderCompleteHandler'];
   
     return $events;
   }
   
   /**
    * Reduces the stock value after purchase.
    *
    * @param \Drupal\state_machine\Event\WorkflowTransitionEvent $event
    *   Event object.
    */
    public function orderCompleteHandler(WorkflowTransitionEvent $event) {
      /** @var \Drupal\commerce_order\Entity\OrderInterface $order */
      $order = $event->getEntity();
      // Order items in the cart.
      $items = $order->getItems();
   
      foreach ($items as $key => $item) {
        $entity = $item->getPurchasedEntity();
        $quantity = $item->getQuantity();
        $stock_field_value = $entity->field_stock->value;
        $entity->field_stock->value = $stock_field_value - $quantity;
        $entity->save();
      }
    }
   

3. Then I have a stock checker which implements AvailabilityCheckerInterface.

   namespace Drupal\commerce_custom_stock;
   
   use Drupal\commerce\Context;
   use Drupal\commerce_order\AvailabilityCheckerInterface;
   use Drupal\commerce_order\AvailabilityResult;
   use Drupal\commerce_order\Entity\OrderItemInterface;
   
   /**
    * Class to check stock.
    */
   class VariationAvailabilityChecker implements AvailabilityCheckerInterface {
   
     /**
      * {@inheritdoc}
      */
     public function applies(OrderItemInterface $order_item) {
       return TRUE;
     }
   
     /**
      * {@inheritdoc}
      */
     public function check(OrderItemInterface $order_item, Context $context) {
       $entity = $order_item->getPurchasedEntity();
       $quantity = $order_item->getQuantity();
       // Order should not be placed if Stock is zero.
       if ($entity->field_stock->value <= 0) {
         $result = t('This product is out of stock.');
         return AvailabilityResult::unavailable($result);
       }
       // Cart quantity should not be more than stock value.
       if ($quantity > $entity->field_stock->value) {
         $result = t('The quantity is more than the stock quantity');
         return AvailabilityResult::unavailable($result);
       }
   
       return AvailabilityResult::neutral();
     }
   
   }
   
   

Two customers add the same product in the cart; one of them purchased and finished the stock. If the person tries to purchase it, a 403 error is shown. I want to show some message instead of the 403 error.

Is there any way to achieve this?