rkt: check if container was already terminated

[Mic92]

$ ./target/debug/cntr attach 5e020cea
no suitable container found, got the following errors:
  - ProcessId: not a valid pid: `5e020cea`: invalid digit found in string
  - Rkt: could not find container process belonging to rkt container '5e020cea': no child process found for pid 20849
  - Docker: Failed to list containers. 'docker inspect --format '{.State.Running};{.State.Pid}' 5e020cea' exited with exit code: 1: Error: No such object: 5e020cea
  - Nspawn: Failed to list containers. 'machinectl show --property=Leader 5e020cea' exited with exit code: 1: Could not get path to machine: No machine '5e020cea' known
  - Lxc: Failed to list containers. 'lxc-info --no-humanize --pid --name 5e020cea' exited with exit code: 1: 5e020cea doesn't exist

currently the error message is not too helpful.