allow installation exception to be caught in InstallCommand

Fixes #794.

### Before
(with exit code 0)

![image](https://cloud.githubusercontent.com/assets/677704/3566233/01d8a332-0aec-11e4-9feb-3aae470a4e09.png)

### After
(with exit code 1)

![image](https://cloud.githubusercontent.com/assets/677704/3566236/11774de8-0aec-11e4-9885-12dd2a603844.png)