Keep the AS list when the asupdate generator fails (closes #173)

make asupdate piped the generator into gzip, so the target saw gzip's
exit status. A failed download replaced pkg/asinfo/asdata.json.gz with
an empty file and the target still exited 0.

The target now runs the new script/asupdate. It writes the output of
cmd/asinfo-gen to a temporary file. Only when the generator succeeds
does it compress the list and move it into place. Otherwise it leaves
the file alone and exits non-zero. The generator now fails when as.csv
holds no AS.

The generator reads as.csv from ipverse/as-metadata, the source's new
name. The list it builds there is unchanged, so no new list is
committed.

Model: opus-5-5

cmd/asinfo-gen/main_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"slices"
 	"strings"
 	"testing"
@@ -53,3 +54,20 @@ func TestParseCSVRejectsAnotherHeader(t *testing.T) {
 		}
 	}
 }
+
+// TestParseCSVRejectsAFileWithNoASes parses files that hold no AS, one with
+// only the header and one whose only record has no valid AS number. It
+// expects errNoASes, so that make asupdate keeps the old list.
+func TestParseCSVRejectsAFileWithNoASes(t *testing.T) {
+	t.Parallel()
+
+	for _, asCSV := range []string{
+		"asn,handle,description,country-code\n",
+		"asn,handle,description,country-code\nAS1,LVLT-1,Level 3,US\n",
+	} {
+		_, err := parseCSV(strings.NewReader(asCSV))
+		if !errors.Is(err, errNoASes) {
+			t.Errorf("parseCSV(%q) = %v, want %v", asCSV, err, errNoASes)
+		}
+	}
+}