fixes #3624 - raise exception when hook fails

Previously, failure of a hook was silent in the GUI: no error bubble.   If foreman_hook raises, the error is correctly generated:

![screen shot 2013-11-15 at 5 56 07 pm](https://f.cloud.github.com/assets/429763/1551654/9e22c30e-4e17-11e3-9b6d-77e4d56d41a2.png)

I'm not sure if this the erhm, "rubonic" way to do this, though.