fix(netxlite): map servfail error (#728)

This error occurred for example when querying kazemjalali.com
in websteps measurements run from Iran.

This error is relatively uncommon, but it still makes sense to
create a specific mapping rule for it.

Originally: 4269e82fbd

See https://github.com/ooni/probe/issues/2096

internal/netxlite/dnsdecoder_test.go

@@ -47,6 +47,18 @@ func TestDNSDecoder(t *testing.T) {
 			}
 		})
 
+		t.Run("Servfail", func(t *testing.T) {
+			d := &DNSDecoderMiekg{}
+			data, err := d.DecodeLookupHost(
+				dns.TypeA, dnsGenReplyWithError(t, dns.TypeA, dns.RcodeServerFailure))
+			if !errors.Is(err, ErrOODNSServfail) {
+				t.Fatal("not the error we expected", err)
+			}
+			if data != nil {
+				t.Fatal("expected nil data here")
+			}
+		})
+
 		t.Run("no address", func(t *testing.T) {
 			d := &DNSDecoderMiekg{}
 			data, err := d.DecodeLookupHost(dns.TypeA, dnsGenLookupHostReplySuccess(t, dns.TypeA))