Exit with non-zero for unknown commands.

If you accidentally attempt to invoke a command that is not available you receive a nice warning message like the following:

```
The specified command asdfasdfasdf is invalid, for available options see ember help.
```

This comes from the `UnknownCommand` in `lib/cli/lookup-command.js` [here](https://github.com/stefanpenner/ember-cli/blob/master/lib/cli/lookup-command.js#L11).  Instead of using `this.ui.writeLine` we should throw a `SilentError`.