Merge #13149: Handle unsuccessful fseek(...):s

Summary:
20ce5af Print a log message if we fail to shrink the debug log file (practicalswift)
29c9bdc Handle unsuccessful fseek(...):s (practicalswift)

Pull request description:

Handle unsuccessful `fseek(...)`:s.

**Note to reviewers:** What is the most appropriate course of actions for each of these unsuccessful `fseek(...)`:s?

Tree-SHA512: 5b3d82dbdd15d434d3f08dcb4df62888da4df8541d2586f56a4e529083005f6782c39e10645acd1ec403da83061bbfd8dbf2dddc66e09268d410ad0918c61876

Backport of Core PR13149
https://github.com/bitcoin/bitcoin/pull/13149/

Test Plan:

make check
test_runner.py

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

Reviewed By: deadalnix, Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4021