Enhancement: Print JSON-formatted errors during `restore --json`

Restic used to print any `restore` errors directly to the console as freeform
text messages, even when using the `--json` option.

Now, when `--json` is specified, restic prints them as JSON formatted messages.

https://github.com/restic/restic/issues/4944
https://github.com/restic/restic/pull/4946
