index: Generalize logged statements in BaseIndex.

Summary:
backported fourth commit (f376a4924) from PR13243
https://github.com/bitcoin/bitcoin/commit/f376a4924

Depends on D2935

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox, Fabien

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T630

Differential Revision: https://reviews.bitcoinabc.org/D2936