Steps to reproduce:

1. Log in to storefront.
2. Add a product to cart.
3. Go to cart view.
4. In quantity input provide a letter or special character sign
5. Verify web browser console output.

Actual result:

See attached screenshots

An error 500 occurs when user fill in quantity input by a letter or special character sign.

500 (Internal Server Error) 

{
    "ErrorMessage": {
        "_media-type": "application\/vnd.ibexa.api.ErrorMessage+json",
        "errorCode": 500,
        "errorMessage": "Internal Server Error",
        "errorDescription": "Requested amount must be grater or equal than 1 but 0 given.",
        "trace": "#0 
       }
}

Expected result:

Validation works correctly, user cannot fill in QTY input by letter or special character signs.