Don't show a big stack on failure

Throws a PluginError in the fail reporter, so that when the validation fails, the output looks like that:

![screen shot 2014-08-27 at 8 16 22 pm](https://cloud.githubusercontent.com/assets/1530203/4064997/d25c9c0a-2e16-11e4-8fc8-df1fcaded781.png)

instead of that:

![screen shot 2014-08-27 at 8 15 31 pm](https://cloud.githubusercontent.com/assets/1530203/4065003/df47c32c-2e16-11e4-88b9-cbd28583802e.png)