Spec #164: A sixth outcome word: not_found splits out of errors

backend/internal/latest/read.go

@@ -25,9 +25,12 @@ type seriesRead struct {
 // errChallengeHeld (browser.go) is the outcome of a Site that answered with
 // its interstitial — status 403 (cf-mitigated) or a challenge page body — and
 // is how a Lane tells a refusal from an ordinary failure (issue #100).
+// errNotFound marks any other 4xx: the page is gone — a fact the owner can act
+// on — as distinct from a Site or database that is merely unwell (issue #164).
 var (
 	errNotFetchable = errors.New("series url not fetchable")
 	errNoFetcher    = errors.New("no fetcher for site")
+	errNotFound     = errors.New("series page not found")
 )
 
 // readSeriesPage performs the series-page read the poll and the acquirer have
@@ -59,6 +62,9 @@ func readSeriesPage(ctx context.Context, site, seriesURL string, browser, tls Fe
 		return seriesRead{}, fmt.Errorf("%w: fetch %s: status %d", errChallengeHeld, seriesURL, status)
 	}
 	if status != 200 {
+		if status >= 400 && status < 500 {
+			return seriesRead{}, fmt.Errorf("%w: fetch %s: status %d", errNotFound, seriesURL, status)
+		}
 		return seriesRead{}, fmt.Errorf("fetch %s: status %d", seriesURL, status)
 	}
 	if isInterstitial(body) {