updating an sequence without an actual sequence fails in a surprising way

@rabbah commented on [Fri Apr 07 2017](https://github.com/apache/incubator-openwhisk/issues/2121)

```
wsk action create a --docker a
wsk action create b --docker b
wsk action create s a,b --sequence
wsk action update s --sequence
```

The CLI accepts the update with no artifact present and constructs the following incorrect request to post to the API:

```
{{"namespace":"_","name":"s","exec":{"kind":"sequence","components":["/_/"]}}
```

The controller will reject this because the component has an invalid action name.
```
requirement failed: The fully qualified name of the entity must contain at least the namespace and the name of the entity.
```

---

@dubeejw commented on [Fri Jul 14 2017](https://github.com/apache/incubator-openwhisk/issues/2121#issuecomment-315459951)

@rabbah, is this issue still valid? Here's the output I get with the current CLI:
```
$ wsk action create a --docker a
ok: created action a
$ wsk action create b --docker b
ok: created action b
$ wsk action create s a,b --sequence
ok: created action s
$ wsk action update s --sequence
error: Invalid argument(s). An action name and code artifact are required.
Run 'wsk --help' for usage.
```

---

@rabbah commented on [Fri Jul 14 2017](https://github.com/apache/incubator-openwhisk/issues/2121#issuecomment-315460699)

that's better.