fix(ssh/NewTLSSSHClient): add error handling

````
% ~/src/fleet/bin/fleetctl ssh 6e4a0139 sudo reboot
The authenticity of host '0.0.0.0:0' can't be established.
RSA key fingerprint is b0:9f:78:c3:9d:98:03:28:76:b3:51:d7:d2:15:e9:79.
Are you sure you want to continue connecting (yes/no)?
```
If I answer no here, fleetctl will panic.
This PR fixes this.

Get idea from #230