An AggregateResponse is the output result version of an AggregateRequest
It contains ResponseMeta and a sequence of PartResponses
Look up a PartResponse by id.
Look up the content of a PartResponse by id.
Note that if the API returned an error response (e.g. a 500 Internal Server Error),
then the part contents will probably be some kind of error message, which you probably
don't want to show to an end-user.
It's safer to call findPart and check the HTTP status code before using the contents.