Handle 500 from stopping a container.

There appears to be a race condition between

- a process dying, a SIGCHLD being sent, and docker processing that signal,
  marking the container as stopped
and
- docker receiving a request to kill a container, checking that the container
  is still alive, and sending the signal.

This partially reverts commit c9a58bdc262d410f90588a3bd0a9dd18feb4bec8.