If something goes wrong while processing the request, an appropriate HTTP error code is returned - for example 400 Bad Request or 500 Internal server error.
In addition, the response contains an errors array, containing error messages - usually just one, but it can contain more in more complex scenarios - multiple validation errors for example.
In some cases, for example tax calculation, even when an error occurs, the response might contain additional information - for example country detected from an IP address. Please consult individual endpoint for detailed information.
See Calculate tax and Store transaction for example responses.