docker kill should return error if container is not running.

Assuming that docker kill is trying to actually kill the container
is a mistake.  If the container is not running we should report it
back to the caller as a error.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Docker-DCO-1.1-Signed-off-by: Regan McCooey <rmccooey27@aol.com> (github: rmccooey27)

Docker-DCO-1.1-Signed-off-by: Regan McCooey <rmccooey27@aol.com> (github: rhatdan)