It’s Better Errors GEM. It will handle the error and we can see exactly where the error has occurred in development machine.

GITHUB : https://github.com/charliesome/better_errors

Steps to configure in our rails3 application

1. In your Gemfile add

group :development do
  gem "better_errors"
end

If you would like to use Better Errors’ advanced features you need to add the binding_of_caller

group :development do
  gem "better_errors"
  gem "binding_of_caller"
end

2. Add in below line at the end config/environments/development.rb 

   BetterErrors::Middleware.allow_ip! ENV['TRUSTED_IP'] if ENV['TRUSTED_IP']

3. Thats it!
   TRUSTED_IP=127.0.0.1 rails s