Backport VerifyResult enum class
Summary:
Partial backport of Core PR 10742 (VerifyResult only)
https://github.com/bitcoin/bitcoin/pull/10742/files
Progress towards T549
Test Plan: ninja check
Reviewers: deadalnix, Fabien, #bitcoin_abc
Reviewed By: Fabien, #bitcoin_abc
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2718