Improvement: standard "Object" property in all API responses

Currently audio and image endpoints are missing the “object” property in returned JSON.

Would be good to have them to simplify JSON to class mapping in clients when dealing with large and mixed batches processing.

Good thing would be to have the error object described in docs and have “object” property as well for responses where body has error details.

Currently have to keep track of all requests in a batch as a backup to check the response type if that “object” property is missing…