Add a "theme doesn't exists" error.
Found this script a while ago, seems like a nice alternative to Jekyll (node.js + grunt <3).

Anyway I just tried to create a blog and spelled wrong on the theme name. The script just hang and even ctrl+c didn't work. Had been great if I instead had got an error message like "Theme doesn't exists".

For example:

```
$_ cabin new blog theme-path-that-doest/exists

>> Error: the theme "theme-path-that-doest/exists" doesn't exists.
```
