Serving generic image for failed requests

@drsnyder @garthwebb 
https://wikia-inc.atlassian.net/browse/VID-2006
Serves up public/brokenImage.jpg when a request fails (500-level) or original image is not found (400-level).

In the case of a 400-level error, brokenImage.jpg is thumbnailed just how the original would have been thumbnailed. Doesn't make sense to do that on a 500-level error, though, since if the thumbnail of brokenImage.jpg fails (for the same reasons the original thumbnailing attempt fails), we'll end up in an endless loop of failed thumb -> try to thumb brokenImage -> failed thumb -> try to thumb brokenImage -> etc
Should we add any tests for `vignette.util.image-response`?