I tried to extract review requests using parts of the post-review code. Whenever I requested a request ID which is _not_ available on Review Board I got a HTTP404 exception.
In my opinion this is unhandy because there seems to be no other way available to obtain the review requests from the server. I'm forced to poll for each id and it is quite usual that some do not exist.

I would like to change the review_request API to return INVALID_CHANGE_NUMBER or DOES_NOT_EXIST if the review object cannot be found.

Side info: In post-review I'm using get_review_request which calls to api/json/reviewrequests/<RID>/ to retrieve the review requests.