errors_controler.rb

class ErrorsController < ApplicationController
  def not_found
    # Will render the app/views/errors/not_found.html.erb template
    def not_found
      respond_to do |format|
        format.html { render template: 'errors/not_found', layout: 'layouts/application', status: 404 }
        # Below is to ensure rails doesn't 500 when someone requests /blahblahblah.js or any other non-html format.
        format.all { render nothing: true, status: 404 }
      end
    end
  end

  def error
    render template: 'views/errors/error', layout: 'layouts/application', status: 500
    # Will render the app/views/errors/error.html.erb template
  end
end

routes file

# added as the last routes:

get '404' => 'errors#not_found'
get '500' => 'errors#error'
get '*a' => 'errors#not_found'