new with —dry-run does not create new directory

fixes GH-1592
Why `undefined`?  I assume it is to suppress [this](https://github.com/stefanpenner/ember-cli/blob/master/lib/cli/cli.js#L65) check, but if that is true, I think using the custom `SilentError` (from [here](https://github.com/stefanpenner/ember-cli/blob/master/lib/errors/silent.js)) is a better/easier way to do it.

Basically, this function would change to be:

```javascript
throw new SilentError('Directory \'' + directoryName + '\' already exists.\n');
```