Should provide better error message on broken ubuntu image

[tailhook]

Currently its:

ERROR:vagga::builder: Error building container "ubuntu": step UbuntuRelease { codename: None, version: None, url: Some("https://cloud-images.ubuntu.com/releases/12.04.2/release/ubuntu-12.04-server-cloudimg-amd64.tar.gz"), arch: "amd64", keep_chfn_command: false } failed: Error reading /etc/lsb-release: No such file or directory (os error 2)

The error here, is that you should download -root.tar.gz image. There might be two approaches:

1. List files/folders extracted (this shows that only ubuntu..amd64.img extracted)
2. Match url against regex heuristics

/cc @lilianmoraru