Bugfix: SFTP backend now checks for disk space

A backup to an SFTP backend would spew repeated SSH_FX_FAILURE messages when
the remote disk is full. Restic now reports "sftp: no space left on device"
and exits immediately when it detects this condition.

A fix for this issue was promised in restic 0.12.1, but the fix itself
contained a bug that prevented it from triggering.

https://github.com/restic/restic/issues/3336
https://github.com/restic/restic/pull/3345
https://github.com/restic/restic/pull/4075