Show source view and backtrace on missing template errors

This will help you debug missing template errors, especially if they
come from a programmatic template selection. Thanks to @dhh for
suggesting that.

![screen shot 2014-11-24 at 6 51 48 pm](https://cloud.githubusercontent.com/assets/604618/5168941/bbb8c510-740b-11e4-9d35-d2624b3f9532.png)

The console in the picture is coming from web-console, it won't be available without it included in the Gemfile.