gui: Prompt to reset settings when settings.json cannot be read

Summary:
Currently the GUI shows confusing error messages when settings.json can't be read or written on startup. This causes the unrecoverable read error described in core#21340 and write error described core#21974.

This PR tries to clarify the error dialog, and adds an option to just clear the settings and reset them to default

Fixes core#21340

Co-authored-by: Jarol Rodriguez <jarolrod@tutanota.com>

This is a backport of core-gui#379

Test Plan: ninja && src/qt/bitcoin-qt -settings=/dev/null

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16192