`language plugin list` for bad plugin slugs should throw warning

Hello!

When I type

```
wp language plugin list foobar
```

I always get an output:

![image](https://user-images.githubusercontent.com/3183737/64069210-82284e00-cc45-11e9-92d2-68ed54e3deef.png)

I can write anything I want:

![image](https://user-images.githubusercontent.com/3183737/64069214-8d7b7980-cc45-11e9-805d-516c937973fa.png)

This should not be the case :-)