Always Cancel before Close

Document this contract.

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

task.go

@@ -163,6 +163,7 @@ func (t *task) Start(ctx context.Context) error {
 		ContainerID: t.id,
 	})
 	if err != nil {
+		t.io.Cancel()
 		t.io.Close()
 		return errdefs.FromGRPC(err)
 	}