Transient network errors are poorly handled

[exarkun]

If an SSH connection to a deployment target dies, Morph appears to hang indefinitely, neither retrying nor failing with an error.

I recently left a deploy running and came back hours later only to find:

Pushing paths to storage002:                                                            
        * ...-nixos-system-storage002-19.09pre-git
        * ...-nixos-system-storage002-19.09pre-git.drv                     
[1/104/594 copied (192.0/685.2 MiB)] copying path '...-linux-4.19.79' to 'ssh://storage002'Timeout, server w.x.y.z not responding.

The server was fine but my network hiccuped at some point.

Instead of hanging forever, morph should either actually fail (allowing a morph-invoking driver to implement retry logic) or perform retries itself.