Errors return JSON in this shape: 
{
  "error": {
    "code": "bad_request",
    "message": "invalid request: ..."
  }
}
Common status codes:
  • 400 bad request
  • 401 unauthorized
  • 403 forbidden
  • 404 not found
  • 409 conflict
  • 429 too many requests
  • 500 internal error