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 kept the idea of warning the user (so that he won't worry if he does not see mem/cpu metrics), but with there is no reason to put an additional return because it will stop at the next `for` loop. It is indifferent to me so if you have a good reason I can add a return here.