error if settings.json exists, but is unreadable

Summary: This is a backport of core#22591

Test Plan:
ninja all check-all

$ sudo chmod a-rwx /bitcoinddata/settings.json
$ src/bitcoind
Error: Failed loading settings file:
- /bitcoinddata/settings.json. Please check permissions.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10783