disco init fails on aws lightsail?

[gregsadetsky]

> disco init root@...
[----------------------------------------] 0%    Error: Failed to install Docker
    Error: Failed to run command over SSH
    sudo apt-get update

issue was trying to disco init with "root@" instead of "ubuntu@", which worked - presumably it would be the same problem/incorrect error message with an ec2 machine

this should be better detected/handled