When git-annex-shell p2pstdio fails with 255, it's because the ssh
server is not reachable. Avoid running the fallback action in this case,
since it would just try a second time to connect, and presumably fail.
Note that the closed P2PSshConnection will not be stored in the pool,
so the next request tries again to connect. This is just the right
behavior; when the remote becomes reachable again, the same git-annex
process will start using it.
This commit was sponsored by Ole-Morten Duesund on Patreon.
