better handling of backfill errors on pds fetch

backfill/backfill.go

···
 	err        error
 }
 
+type FetchRepoError struct {
+	StatusCode int
+	Status     string
+}
+
+func (e *FetchRepoError) Error() string {
+	reason := "unknown error"
+	if e.StatusCode == http.StatusBadRequest {
+		reason = "repo not found"
+	} else {
+		reason = e.Status
+	}
+	return fmt.Sprintf("failed to get repo: %s (%d)", reason, e.StatusCode)
+}
+
 // Fetches a repo CAR file over HTTP from the indicated host. If successful, parses the CAR and returns repo.Repo
 func (b *Backfiller) fetchRepo(ctx context.Context, did, since, host string) (*repo.Repo, error) {
 	url := fmt.Sprintf("%s/xrpc/com.atproto.sync.getRepo?did=%s", host, did)
···
 	}
 
 	if resp.StatusCode != http.StatusOK {
-		reason := "unknown error"
-		if resp.StatusCode == http.StatusBadRequest {
-			reason = "repo not found"
-		} else {
-			reason = resp.Status
+		return nil, &FetchRepoError{
+			StatusCode: resp.StatusCode,
+			Status:     resp.Status,
 		}
-		return nil, fmt.Errorf("failed to get repo: %s", reason)
 	}
 
 	instrumentedReader := instrumentedReader{
···
 		r, err = b.fetchRepo(ctx, repoDID, job.Rev(), pdsHost)
 		if err != nil {
 			slog.Warn("repo CAR fetch from PDS failed", "did", repoDID, "since", job.Rev(), "pdsHost", pdsHost, "err", err)
-			return "repo CAR fetch from PDS failed", err
+			rfe, ok := err.(*FetchRepoError)
+			if ok {
+				return fmt.Sprintf("failed to fetch repo CAR from PDS (http %d:%s)", rfe.StatusCode, rfe.Status), err
+			}
+			return "failed to fetch repo CAR from PDS", err
 		}
 	}