Replace integer literals in dnshandle() with an enum class
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

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