MC-26683: Some sort of example how I see the solution of this.

Author: AleksLi

app/code/Magento/GraphQl/Controller/GraphQl.php

@@ -160,6 +160,8 @@ public function dispatch(RequestInterface $request) : ResponseInterface
         } catch (\Exception $error) {
             $result['errors'] = isset($result) && isset($result['errors']) ? $result['errors'] : [];
             $result['errors'][] = $this->graphQlError->create($error);
+            // here we should have data from GraphQlCartInputException
+            $result['data'] = $error->getData();
             $statusCode = ExceptionFormatter::HTTP_GRAPH_QL_SCHEMA_ERROR_STATUS;
         }
 

app/code/Magento/QuoteGraphQl/Model/Cart/AddProductsToCart.php

@@ -8,11 +8,12 @@
 namespace Magento\QuoteGraphQl\Model\Cart;
 
 use Magento\Framework\GraphQl\Exception\GraphQlInputException;
+use Magento\Framework\Message\MessageInterface;
 use Magento\Quote\Api\CartRepositoryInterface;
 use Magento\Quote\Model\Quote;
 
 /**
- * Adding products to cart using GraphQL
+ * Add products to cart
  */
 class AddProductsToCart
 {
@@ -43,16 +44,29 @@ public function __construct(
      *
      * @param Quote $cart
      * @param array $cartItems
+     * @return \Magento\Framework\GraphQl\Exception\GraphQlCartInputException
      * @throws GraphQlInputException
      * @throws \Magento\Framework\Exception\LocalizedException
      * @throws \Magento\Framework\GraphQl\Exception\GraphQlNoSuchEntityException
      */
-    public function execute(Quote $cart, array $cartItems): void
+    public function execute(Quote $cart, array $cartItems): \Magento\Framework\GraphQl\Exception\GraphQlCartInputException
     {
         foreach ($cartItems as $cartItemData) {
             $this->addProductToCart->execute($cart, $cartItemData);
         }
 
+        if ($cart->getData('has_error')) {
+            $e = new \Magento\Framework\GraphQl\Exception\GraphQlCartInputException(__('Shopping cart errors'));
+            $errors = $cart->getErrors();
+            foreach ($errors as $error) {
+                /** @var MessageInterface $error */
+                $e->addError(new GraphQlInputException(__($error->getText())));
+            }
+            $e->addData($cartItems);
+
+            throw $e;
+        }
+
         $this->cartRepository->save($cart);
     }
 }

lib/internal/Magento/Framework/GraphQl/Exception/GraphQlCartInputException.php

@@ -0,0 +1,97 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\Framework\GraphQl\Exception;
+
+use Magento\Framework\Exception\AggregateExceptionInterface;
+use Magento\Framework\Exception\LocalizedException;
+use Magento\Framework\Phrase;
+use GraphQL\Error\ClientAware;
+
+class GraphQlCartInputException extends LocalizedException implements AggregateExceptionInterface, ClientAware
+{
+    const EXCEPTION_CATEGORY = 'graphql-input';
+
+    /**
+     * @var boolean
+     */
+    private $isSafe;
+
+    /**
+     * The array of errors that have been added via the addError() method
+     *
+     * @var \Magento\Framework\Exception\LocalizedException[]
+     */
+    private $errors = [];
+
+    /**
+     * @var array
+     */
+    private $data = [];
+
+    /**
+     * Initialize object
+     *
+     * @param Phrase $phrase
+     * @param \Exception $cause
+     * @param int $code
+     * @param boolean $isSafe
+     */
+    public function __construct(Phrase $phrase, \Exception $cause = null, $code = 0, $isSafe = true)
+    {
+        $this->isSafe = $isSafe;
+        parent::__construct($phrase, $cause, $code);
+    }
+
+    /**
+     * @inheritdoc
+     */
+    public function isClientSafe() : bool
+    {
+        return $this->isSafe;
+    }
+
+    /**
+     * @inheritdoc
+     */
+    public function getCategory() : string
+    {
+        return self::EXCEPTION_CATEGORY;
+    }
+
+    /**
+     * Add child error if used as aggregate exception
+     *
+     * @param LocalizedException $exception
+     * @return $this
+     */
+    public function addError(LocalizedException $exception): self
+    {
+        $this->errors[] = $exception;
+        return $this;
+    }
+
+    /**
+     * Get child errors if used as aggregate exception
+     *
+     * @return LocalizedException[]
+     */
+    public function getErrors(): array
+    {
+        return $this->errors;
+    }
+
+    /**
+     * @param array $data
+     * @return GraphQlInputException
+     */
+    public function addData(array $data): self
+    {
+        $this->data = $data;
+        return $this;
+    }
+}