Return 404 on exec-inspect when container is dead but exec is still around

When a container is removed but it had an exec, that still hasn't been
GC'd per PR #14476, and someone tries to inspect the exec we should
return a 404, not a 500+container not running.  Returning "..not running" is
not only misleading because it could lead people to think the container is
actually still around, but after 5 minutes the error will change to a 404
after the GC. This means that we're externalizing our internall soft-deletion/GC
logic which shouldn't be any of the end user's concern. They should get the
same results immediate or after 5 minutes.

Signed-off-by: Doug Davis <dug@us.ibm.com>