when FORMERR comes, differentiate based on OPT
In particular, non-support of EDNS is implied iff FORMERR without OPT comes. If OPT is there, one possibility is that there was something wrong in the OPT that we sent, but it seems much more likely that this particular server is just bad and we want to try another one. https://tools.ietf.org/html/rfc6891#section-7 In particular, we would be in trouble if we dropped OPT in a zone that is covered by DNSSEC.