Fix for #794 - failed installation yields exit code 1

Fixes #794.

### Before
(with exit code 0)

![image](https://cloud.githubusercontent.com/assets/677704/3566280/61e0b808-0aee-11e4-895f-68868e35a9e6.png)

### After
(with exit code 1)

![image](https://cloud.githubusercontent.com/assets/677704/3566286/7118c3ec-0aee-11e4-9755-d8b07e7c5691.png)