watcher: names say "usable reply", as their comments do (rework for #302)

The function that decides the round could not reach the DNS, and its
test, are renamed to say no root server gave a usable reply, since a
root server's address that replied unusably counts the same.

Model: opus-5-5

internal/watcher/watcher.go

@@ -299,16 +299,16 @@ func (w *Watcher) runDNSChecks(ctx context.Context) {
 	}
 
 	if ctx.Err() == nil {
-		w.dnsUnreachable.Store(noRootServerReplied(errs))
+		w.dnsUnreachable.Store(noRootServerGaveUsableReply(errs))
 	}
 }
 
-// noRootServerReplied reports whether errs, the errors of the lookups in
-// a round of DNS checks, nil for a lookup that succeeded, show that
-// every lookup failed because no root server gave a usable reply: each
-// is ErrNoRootServerAnswered or ErrIntercepted. A round with no lookups
-// shows nothing.
-func noRootServerReplied(errs []error) bool {
+// noRootServerGaveUsableReply reports whether errs, the errors of the
+// lookups in a round of DNS checks, nil for a lookup that succeeded,
+// show that every lookup failed because no root server gave a usable
+// reply: each is ErrNoRootServerAnswered or ErrIntercepted. A round with
+// no lookups shows nothing.
+func noRootServerGaveUsableReply(errs []error) bool {
 	for _, err := range errs {
 		if !errors.Is(err, resolver.ErrNoRootServerAnswered) &&
 			!errors.Is(err, resolver.ErrIntercepted) {