Error messages in LoadBlockIndexGuts() use func instead of hardcoding function name.

Summary:
Partial backport of Core PR9176
https://github.com/bitcoin/bitcoin/pull/9176/files

Other parts are irrelevant because we have config.

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3677