The exception_handler gem is a great gem for easily integrating custom error pages in a Rails application (instead of the ugly grey screen of death). However the Gem page itself is a little confusing, so here are the steps I use.
Add this to the `Gemfile`:
gem "exception_handler", "~0.4"
Then run the installer commands:
And that's literally it. From there you can edit the view files the gem provides.