Render custom view when 500 error is encountered. Attempts to fix #1628.

This is an initial attempt at fixing the 500 error issue. In development, I had to disable :show_exceptions in the Sinatra configure block. (This has been commented out for production.) I'm not sure if this implementation will interfere with Bugsnag's error capturing.

![500](https://cloud.githubusercontent.com/assets/2092395/3518793/fd9b17be-0708-11e4-86f4-4672d009bd62.png)