Review and backport EOS PR 3277 reflector_verifier_visitor

https://github.com/EOSIO/eos/pull/3277

Probably low priority.

Probably we have more interest in the code about handling of exceptions in `exception::to_string`:
https://github.com/EOSIO/eos/commit/1cfb4365bc60fccd144a54a5f53188a329bd15d2, note: there is another patch after that: https://github.com/EOSIO/eos/commit/eb85fcc949915ee3efae7a7b53b5db8c0b4affa0