Handling errors with proper JSON responses.

Implementation inspired by...
- https://github.com/visionmedia/express/blob/master/examples/error/index.js
- http://www.hacksparrow.com/express-js-custom-error-pages-404-and-500.html

Concept inspired by...
- http://soabits.blogspot.com/2013/05/error-handling-considerations-and-best.html
Merge pull request #102 from apiaryio/better-error-handling

Handling errors with proper JSON responses