http://railscasts.com/episodes/53-handling-exceptions-revised

Reasoning

• Almost every Rails app I know leaves default error pages, which is a shame.
• In my experience when it comes to implementing custom error pages they are written in plain html and put in public, and after that completely forgotten, differing more and more from layout design.
• For 500 API calls, the public/500.html is rendered by default. What should render is JSON with error message.

The solution is to define explicit controller for handling exceptions. That way you can use site layout template, SLIM, rails helpers etc. In general error pages become more dynamic.

Possible downsides

Error handling deadloop (error is thrown from error controller). I'll try to implement some safeguard for this case though. I think of fallback to public/500.html when there is an exception thrown in errors controller.

Also, to prevent such cases I think ExceptionsController should inherit directly from ActionController::Base, instead of ApplicationController. That should protect it from programming errors committed in ApplicationController.