validation: Change return value of VerifyDB to enum type

Summary:
This does not change behavior. It is in preparation for
special handling of the case where VerifyDB doesn't finish
for various reasons, but doesn't fail.

This is a backport of core#25574
https://github.com/bitcoin/bitcoin/pull/25574/commits/6360b5302d2675788de5c4a28ea77d823f6d809e
Depends on D14925

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14926