Review and backport EOS PR 3277 reflector_verifier_visitor

[abitmore]

EOSIO/eos#3277

Probably low priority.

Probably we have more interest in the code about handling of exceptions in exception::to_string:
EOSIO/eos@1cfb436, note: there is another patch after that: EOSIO/eos@eb85fcc

[pmconrad]

Done with bitshares/bitshares-fc#167 + bitshares/bitshares-fc#168 , waiting for merge + fc bump

[pmconrad]

The cherry-picked changes are covered by the EOS license (MIT).