It would be great to have not just message as response when request is not successful.
For example adding product to cart
response 400 Bad request and message "You need to choose options for your item."
or 400 Bad request and message "This product is out of stock."
or 400 Bad request and message "We don't have as many \"Chaz Kangeroo Hoodie-XS-Gray\" as you requested."
Compare messages to identify error is not good approach... even when you want to connect to more magento websites with different languages...