Fix error on doulbe Kill calls

This returns a typed error for calls to Kill when the process has
already finished.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

container_test.go

@@ -665,3 +665,72 @@ func TestDeleteRunningContainer(t *testing.T) {
 	}
 	<-statusC
 }
+
+func TestContainerKill(t *testing.T) {
+	if testing.Short() {
+		t.Skip()
+	}
+	client, err := New(address)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer client.Close()
+
+	var (
+		ctx, cancel = testContext()
+		id          = t.Name()
+	)
+	defer cancel()
+
+	image, err := client.GetImage(ctx, testImage)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	spec, err := GenerateSpec(WithImageConfig(ctx, image), WithProcessArgs("sh", "-c", "cat"))
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	container, err := client.NewContainer(ctx, id, WithSpec(spec), WithImage(image), WithNewRootFS(id, image))
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	defer container.Delete(ctx)
+
+	task, err := container.NewTask(ctx, Stdio)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	defer task.Delete(ctx)
+
+	statusC := make(chan uint32, 1)
+	go func() {
+		status, err := task.Wait(ctx)
+		if err != nil {
+			t.Error(err)
+		}
+		statusC <- status
+	}()
+
+	if err := task.Start(ctx); err != nil {
+		t.Error(err)
+		return
+	}
+	if err := task.Kill(ctx, syscall.SIGKILL); err != nil {
+		t.Error(err)
+		return
+	}
+	<-statusC
+
+	err = task.Kill(ctx, syscall.SIGTERM)
+	if err == nil {
+		t.Error("second call to kill should return an error")
+		return
+	}
+	if err != ErrProcessExited {
+		t.Errorf("expected error %q but received %q", ErrProcessExited, err)
+	}
+}