ssh failure conditions never propagate to callers

[nbastin]

In cases where your sliver is messed up (which seems to happen a lot), fablib silently ignores all ssh failure conditions and never tells the user about them (other than returning None from Node/Interface methods).

For example this happens when I try to ssh directly to a broken node in a slice (other nodes work fine):

$ ssh -F ~/.ssh/fabric_ssh_config ubuntu@2001:18e8:fff0:3:f816:3eff:fe8a:4074
channel 0: open failed: connect failed: No route to host
stdio forwarding failed
kex_exchange_identification: Connection closed by remote host
Connection closed by UNKNOWN port 65535

but this never propagates to a caller of a fablib method, so I can't tell the difference between something that hasn't yet been configured and a node that is unreachable.