Function json_error_response_with_fields 

pub fn json_error_response_with_fields(
    status: StatusCode,
    code: &str,
    message: &str,
    extra_fields: &[(String, String)],
) -> (StatusCode, String, Bytes)

Build an AWS JSON error response carrying additional top-level members (e.g. DynamoDB’s ConditionalCheckFailedException.Item). Each field value is parsed as JSON when possible (so an object/array is embedded as-is) and otherwise emitted as a JSON string.