Bugfix: Improve error handling in the `restore` command

The `restore` command used to not print errors while downloading file contents
from the repository. It also incorrectly exited with a zero error code even
when there were errors during the restore process. This has all been fixed and
`restore` now returns with a non-zero exit code when there's an error.

https://github.com/restic/restic/issues/3166
https://github.com/restic/restic/pull/3207
