User is trying to add a product from third party to Magento store. The product has been already created in Magento and prod id is passed in third party page. Using event-observer, the product is being added using below code from third part and redirected to Magento cart page.

Module A 

events.xml

<event name="controller_action_predispatch_mymodule_result_addproduct">      <observer name="addproduct_display" instance="Vendor\ModuleA\Observer\ThirdPartyAdd" /> </event> 

ThirdPartyAdd.php

namespace Vendor\ModuleA\Observer;  class ThirdPartyAdd implements \Magento\Framework\Event\ObserverInterface {  public function __construct(             \Magento\Store\Model\StoreManagerInterface $  storeManager,              \Magento\Checkout\Model\Cart $  cart,              \Magento\Catalog\Model\Product $  product,              \Magento\Quote\Model\QuoteFactory $  quote,              \Magento\Catalog\Model\ProductFactory $  productFactory,              \Magento\Framework\App\ResponseFactory $  responseFactory,              \Magento\Framework\UrlInterface $  url,             \Magento\Catalog\Model\ResourceModel\Product\CollectionFactory $  productCollectionFactory,             \Magento\Eav\Model\Config $  eavConfig     ) {         $  this->_storeManager = $  storeManager;         $  this->_cart = $  cart;         $  this->_product = $  product;         $  this->quote = $  quote;         $  this->_productFactory = $  productFactory;         $  this->_responseFactory = $  responseFactory;         $  this->_url = $  url;         $  this->_productCollectionFactory = $  productCollectionFactory;         $  this->eavConfig = $  eavConfig;     }      public function execute(\Magento\Framework\Event\Observer $  observer) {      if (isset($  _POST['source']) && $  _POST['source'] == 'third party') {              for ($  i = 0; $  i < sizeof($  _POST['prod_id']); $  i++) {                  $  productModel = $  this->_productFactory->create();                 $  product = $  productModel->load($  _POST['prod_id'][$  i]);                  $  product_type = $  product->getTypeID();                  if ($  product_type == 'simple') {                     $  params = array(                         'qty' => $  _POST['prod_qty'][$  i],                     );                     $  this->_cart->addProduct($  product, $  params);                     $  this->_cart->save();                 }                 if ($  product_type == 'configurable') {                      $  simple_products = $  product->getTypeInstance()->getUsedProducts($  product);                     $  attributes = $  product->getTypeInstance(true)->getConfigurableAttributesAsArray($  product);                      $  attr_label = explode('-', $  _POST['prod_attr'][$  i]);                      foreach ($  simple_products as $  simple_product) {                         $  super_attributes = array();                         foreach ($  attributes as $  attribute) {                             foreach ($  attribute['values'] as $  value) {                                  if(in_array($  value['label'], $  attr_label)){                                     echo $  value['label'];                                     $  super_attributes[$  attribute['attribute_id']] = $  value['value_index'];                                 }                             }                         }                     }                      $  this->_cart->addProduct($  product, array(                         'qty' => $  _POST['prod_qty'][$  i],                         'super_attribute' => $  super_attributes                     ));                      $  this->_cart->save();                 }             }              $  customerBeforeAuthUrl = $  this->_url->getUrl('checkout/cart');             $  this->_responseFactory->create()->setRedirect($  customerBeforeAuthUrl)->sendResponse();             die();         }      } 

The issue is product quantity exceeds, it shows the exception We don't have as many product name as you requested and stops the execution.

How do I show this exception message on Magento cart page ?