Make every failed check and unreadable counter fatal instead of degrading

main.go

@@ -211,11 +211,14 @@ func lookupEndpoint(name string) (endpoint, error) {
 	}
 	var mac [6]byte
 	copy(mac[:], ifi.HardwareAddr)
-	e := endpoint{name: name, idx: ifi.Index, mac: mac, mtu: ifi.MTU}
-	if v, ok := readUint("/sys/class/net/" + name + "/speed"); ok {
-		e.speed = float64(v) / 1000
+	// The rate columns are graded against this, so a guessed speed would silently
+	// grade every reading against the wrong target.
+	v, ok := readUint("/sys/class/net/" + name + "/speed")
+	if !ok || v == 0 {
+		return endpoint{}, fmt.Errorf("%s: cannot read link speed", name)
 	}
-	return e, nil
+	return endpoint{name: name, idx: ifi.Index, mac: mac, mtu: ifi.MTU,
+		speed: float64(v) / 1000}, nil
 }
 
 func parseSizes(s string) ([]int, error) {
@@ -707,9 +710,6 @@ func run(aName, bName, sizesArg string,
 		[]bool{false, false, false, true, true}, linkRows))
 
 	target := a.speed
-	if target <= 0 {
-		target = 10
-	}
 	sizeStrs := make([]string, len(sizes))
 	for i, s := range sizes {
 		sizeStrs[i] = fmt.Sprintf("%d", s)