Do we know for sure that we'll only ever get at most one `error` event (either here or from an entry stream)? That seems plausible, but if it's possible to get multiple (e.g. maybe both the entry and the main read stream could emit an error), we should check `callbackCalled` in these places as well to make sure we don't call back more than once. (Dunno if anything particularly bad would happen from doing so, but seems worth avoiding.)