What does this PR change? What problem does it solve?

The Load and List operations of the sftp backend did not explicitly check whether the server connection is broken. This causes restic to retry "connection lost" errors for those operations.

Was the change previously discussed in an issue or on the forum?

Showed up in https://forum.restic.net/t/restic-hanging-on-backup/8559/2

Checklist

• I have read the contribution guidelines.
• I have enabled maintainer edits.
• I have added tests for all code changes.
• I have added documentation for relevant changes (in the manual).
• There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• I have run gofmt on the code in all commits.
• All commit messages are formatted in the same style as the other commits in the repo.
• I'm done! This pull request is ready for review.