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.go

@@ -22,6 +22,8 @@ func (d *DNSDecoderMiekg) parseReply(data []byte) (*dns.Msg, error) {
 		return nil, ErrOODNSNoSuchHost
 	case dns.RcodeRefused:
 		return nil, ErrOODNSRefused
+	case dns.RcodeServerFailure:
+		return nil, ErrOODNSServfail
 	default:
 		return nil, ErrOODNSMisbehaving
 	}