dirmngr: Make WKD_GET work even for servers not handling SRV RRs.

* dirmngr/server.c (proc_wkd_get): Take care of DNS server failures

Unfortunately there are resolver setups which don't handle SRV records
but return a server error. We let a not found error pass, because
that merely means the domain does not exists.

• GnuPG-bug-id: T4729