Less Exception with Docker check

Aims at fixing #980 and #981 

* Warning if no containers instead of exception
* Log if fail to open a cgroup file but keep going. If we fail to open them all, then raise the Docker 0.9/0.10 bug Exception.
I think we should explicity stop the check at that point. It's better than assuming that i will stop because the rest of the code is just a loop on an empty list.