Fix The requested qty is not available" should be received instead of Internal server error

[XxXgeoXxX]

Original issue: #350

Manual testing scenarios (*)

Steps to reproduce (*)

1. Create Configurable Product with
   Name & SKU: conf
   Weight: 1
   Price: 1
   Variations:

• sku: conf-white, qty: 12, price: 10
• sku: conf-black, qty: 12, price: 10

1. Perform GraphQL query:

mutation {
  addConfigurableProductsToCart(
    input: {
      cart_id: "gbK3g1pcEiyd39h2clKomLU6qsyFLAVT"
      cartItems: [
        {
          variant_sku: "conf-black"
          data: {
            qty: 13
            sku: "conf-black"
          }
        }
        {
          variant_sku: "conf-white"
          data: {
            qty: 13
            sku: "conf-white"
          }
        }
      ]
    }
  ) {
    cart {
      items {
        id
        qty
        product {
          name
          sku
        }
      }
    }
  }
}

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[naydav]

@XxXgeoXxX Could you cover this case with API-functional tests?
Thanks

[XxXgeoXxX]

API-functional test was added. Please, check and review

[ghost]

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