resolver: error from ResolveIPAddresses when no nameserver answered (closes #190)

ResolveIPAddresses now returns an error wrapping ErrNoNameserverAnswered
when every nameserver of the name's zone timed out or failed, instead of
no addresses and no error. It reads each nameserver's status as the
lookup already sets it; one answer, even NXDOMAIN, is enough for an
empty result without an error. The only caller, the nameserver address
lookup, already keeps the previous addresses on an error; its comment
no longer says the resolver hides this case.

Model: opus-5-5

internal/resolver/export_test.go

@@ -11,6 +11,13 @@ func ExtractRecordValue(rr dns.RR) string {
 	return extractRecordValue(rr)
 }
 
+// CollectIPs exports collectIPs for testing.
+func CollectIPs(
+	results map[string]*NameserverResponse,
+) ([]string, string, error) {
+	return collectIPs(results)
+}
+
 // QueryEachNS exports queryEachNS for testing.
 func (r *Resolver) QueryEachNS(
 	ctx context.Context,