Replace integer literals in dnshandle() with an enum class

Summary:
The error values used in seeder/dns.cpp are defined in RFC1035 under
RCODE. The integer literals currently used as error values can be
replaced with an enum class with accurate names in line with RFC1035.

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4659