MAGETWO-39117: Updating qty that exceeds the qty in stock for registered customer by API should return error

fixed item quantity check

Author: Serhiy Shkolyarenko

app/code/Magento/Quote/Model/Quote/Item/CartItemPersister.php

@@ -72,6 +72,12 @@ public function save(CartInterface $quote, CartItemInterface $item)
                     $item = $quote->updateItem($itemId, $buyRequestData);
                 } else {
                     $currentItem->setQty($qty);
+                    if (!empty($currentItem->getErrorInfos())) {
+                        $errors = $currentItem->getErrorInfos();
+                        /** @var \Magento\Framework\Message\MessageInterface $error */
+                        $errorData = array_shift($errors);
+                        throw new CouldNotSaveException($errorData['message']);
+                    }
                 }
             } else {
                 /** add new item to shopping cart */