So if you get less HTTP bytes than expected, then it’s a HTTP response error and you throw the whole thing away. For example, this sort of situation happens when streaming HTTP. The server first has to send the response headers, which would be a simple 200/206, then the data, which could have a much more complicated code path. If there is an error in that data code path, all you can do is close the connection and trigger an HTTP error since less bytes were delivered than advertised. Client needs to detect this and retry. While this may seem uncommon, this is well understood behavior for HTTP systems.
Or more likely for a range download, you use the bytes you got and keep making further range requests, to get the whole resource in however many tries it takes. And the 403 would come through as soon as you hit an uncached part of the resource.
