Assert validation reasons are contextually correct

Summary: This is a patial backport of Core PR15141 : https://github.com/bitcoin/bitcoin/pull/15141/commits/54470e767bab37f9b7089782b1be73d5883bb244

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6742