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.
Fix typo

alraedy -> already
authors: add Kim, Hirokuni
container: fix typo
container: standardize errors

Now every error uses a pointer receiver.