Merge bitcoin-core/gui#379: Prompt to reset settings when settings.json cannot be read

1ee6d0b01a517893967379677029fb5417978247 gui: Prompt to reset settings when settings.json cannot be read (Russell Yanofsky)

Pull request description:

  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 bitcoin/bitcoin#21340 and write error described bitcoin/bitcoin#21974. Current error read message looks like:

  ![current](https://user-images.githubusercontent.com/7133040/124977362-638ffc80-dffe-11eb-9edd-89135a9bc602.png)

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

  ![new-read-error](https://user-images.githubusercontent.com/7133040/124977636-b669b400-dffe-11eb-8d35-02eda95f48c0.png)
  ![new-read-details](https://user-images.githubusercontent.com/7133040/124977644-bb2e6800-dffe-11eb-9209-11c1c3d7be40.png)

  Additionally the PR also shows a slightly better error message when there is an error trying to write the settings file. This error probably should occur less frequently, but it is easy to improve, and it should be good to make the write error consistent with the read error. The new write error dialog looks like:

  ![new-write-error](https://user-images.githubusercontent.com/7133040/124978016-3bed6400-dfff-11eb-9d79-9b2e9bbc4369.png)

  ![new-write-details](https://user-images.githubusercontent.com/7133040/124978025-3db72780-dfff-11eb-8df5-741f75a402d9.png)

ACKs for top commit:
  jarolrod:
    ACK 1ee6d0b01a517893967379677029fb5417978247
  Zero-1729:
    ACK 1ee6d0b01a517893967379677029fb5417978247
  hebasto:
    ACK 1ee6d0b01a517893967379677029fb5417978247, tested on Linux Mint 20.2 (Qt 5.12.8).

Tree-SHA512: fb57a0a0d032e3f8219fff49a4de69b4c962bf0b448544ccf9d8d4d45c5bd209e23653d4f13300b9e534b9c03de159498bef1658e95defe3ab6a8ecac57d592c

0 files changed, 0 insertions, 0 deletions