Convert ExitStatus to use fn to get details

Instead of requiring callers to read the struct fields to check for an
error, provide the exit results via a function instead which is more
natural.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

cmd/ctr/start.go

@@ -73,14 +73,15 @@ var taskStartCommand = cli.Command{
 		}
 
 		status := <-statusC
-		if status.Err != nil {
+		code, _, err := status.Result()
+		if err != nil {
 			return err
 		}
 		if _, err := task.Delete(ctx); err != nil {
 			return err
 		}
-		if status.Code != 0 {
-			return cli.NewExitError("", int(status.Code))
+		if code != 0 {
+			return cli.NewExitError("", int(code))
 		}
 		return nil
 	},