container: Add ContainerAlreadyRunning and ContainerNotRunning error

Docker returns http.StatusNotFound (304) when trying to (start|stop)
a container that is (already|not) started.

container should return proper error for the HTTP status code.
I think it's better to keep `&` for the consistency.
https://github.com/fsouza/go-dockerclient/pull/140#discussion-diff-16665689L342

Or any particular reasons why we shouldn't return pointer?