Bugfix: Include missing backup error text with `--json`

Previously, when running a backup with the `--json` option, restic failed to
include the actual error message in the output, resulting in `"error": {}`
being displayed.

Restic now includes the error text in JSON output.

https://github.com/restic/restic/issues/4945
https://github.com/restic/restic/pull/4946