Stop asinfo-gen when the as.csv download breaks off (closes #177)

parseCSV skipped a record on any read error and asked for the next one.
When the download of as.csv broke off, the response body returned the
same error on every read, so the generator logged it forever and make
asupdate never finished.

parseCSV now skips only a record that does not parse, a csv.ParseError.
Any other read error ends it with that error, so the generator exits
non-zero and script/asupdate keeps the old list.

Model: opus-5-5

cmd/asinfo-gen/main_test.go

@@ -79,6 +79,36 @@ func TestParseCSVRejectsAFileWithNoASes(t *testing.T) {
 	}
 }
 
+// TestParseCSVSkipsARecordThatDoesNotParse parses an as.csv with a record
+// that has too few fields between two valid records. It expects both valid
+// ASes and no error, so that one bad record does not stop make asupdate.
+func TestParseCSVSkipsARecordThatDoesNotParse(t *testing.T) {
+	t.Parallel()
+
+	const asCSV = "asn,handle,description,country-code\n" +
+		"1,LVLT-1,Level 3 Parent LLC,US\n" +
+		"2,UDEL-DCN\n" +
+		"3,MIT-GATEWAYS,Massachusetts Institute of Technology,US\n"
+
+	got, err := parseCSV(strings.NewReader(asCSV))
+	if err != nil {
+		t.Fatalf("parseCSV: %v", err)
+	}
+
+	want := []ASInfo{
+		{ASN: 1, Handle: "LVLT-1", Description: "Level 3 Parent LLC"},
+		{
+			ASN:         3,
+			Handle:      "MIT-GATEWAYS",
+			Description: "Massachusetts Institute of Technology",
+		},
+	}
+
+	if !slices.Equal(got, want) {
+		t.Errorf("parseCSV = %+v, want %+v", got, want)
+	}
+}
+
 // TestParseCSVStopsWhenTheDownloadBreaksOff parses an as.csv whose download
 // breaks off after the first record. From then on every read fails, as on a
 // broken connection. It expects parseCSV to return that error, so that make