uninitialized constant Spree::Calculator::CouldNotCalculateShipping (NameError)

application.rb

# config/application.rb

require File.expand_path('../boot', __FILE__)

require 'rails/all'

if defined?(Bundler)
  # If you precompile assets before deploying to production, use this line
  Bundler.require(*Rails.groups(:assets => %w(development test)))
  # If you want your assets lazily compiled in production, use this line
  # Bundler.require(:default, :assets, Rails.env)
end

module main_app
  class Application < Rails::Application
    
    config.to_prepare do
      # Load application's model / class decorators
      Dir.glob(File.join(File.dirname(__FILE__), "../app/**/*_decorator*.rb")) do |c|
        Rails.configuration.cache_classes ? require(c) : load(c)
      end

      # Load application's view overrides
      Dir.glob(File.join(File.dirname(__FILE__), "../app/overrides/*.rb")) do |c|
        Rails.configuration.cache_classes ? require(c) : load(c)
      end
    end

    # Settings in config/environments/* take precedence over those specified here.
    # Application configuration should go into files in config/initializers
    # -- all .rb files in that directory are automatically loaded.

    # Custom directories with classes and modules you want to be autoloadable.
    # config.autoload_paths += %W(#{config.root}/extras)

    # Only load the plugins named here, in the order given (default is alphabetical).
    # :all can be used as a placeholder for all plugins not explicitly named.
    # config.plugins = [ :exception_notification, :ssl_requirement, :all ]

    # Activate observers that should always be running.
    # config.active_record.observers = :cacher, :garbage_collector, :forum_observer

    # Set Time.zone default to the specified zone and make Active Record auto-convert to this zone.
    # Run "rake -D time" for a list of tasks for finding time zone names. Default is UTC.
    # config.time_zone = 'Central Time (US & Canada)'

    # The default locale is :en and all translations from config/locales/*.rb,yml are auto loaded.
    # config.i18n.load_path += Dir[Rails.root.join('my', 'locales', '*.{rb,yml}').to_s]
    # config.i18n.default_locale = :de
    ## TODO: Translation fallback for internationalization is should be set to true for production environment
    ##config.i18n.fallbacks = true

    # Configure the default encoding used in templates for Ruby 1.9.
    config.encoding = "utf-8"

    # Configure sensitive parameters which will be filtered from the log file.
    config.filter_parameters += [:password]

    # Enable the asset pipeline
    config.assets.enabled = true

    ## For Heroku to precompile correctly
    #config.assets.initialize_on_precompile = false

    # Version of your assets, change this if you want to expire all your assets
    config.assets.version = '1.0'

    ## Setting default logger to implement unicorn on Heroku
    config.logger = Logger.new(STDOUT)
  end
end

calculator_decorator.rb

# app/models/spree/calculator_decorator.rb

Spree::Calculator.class_eval do
  class Spree::Calculator::CouldNotCalculateShipping < StandardError
  end
 
  class Spree::Calculator::CouldNotValidateOrder < StandardError
  end                      

  class Spree::Calculator::CouldNotCalculateTax < StandardError
  end
  
end 

checkout_controller_decorator.rb

# app/controllers/spree/checkout_controller_decorator.rb

Spree::CheckoutController.class_eval do
 
  rescue_from Spree::Calculator::CouldNotCalculateShipping do |exception|
    flash[:error] = t('controller.spree.checkout.please verify your shipping address') + ' ' + t('controller.spree.checkout.if you continue to receive this error then please contact     us about this issue')
    redirect_to checkout_path
  end
 
  rescue_from Spree::Calculator::CouldNotValidateOrder do |exception|
    flash[:error] = t('controller.spree.checkout.please verify your order') + ' ' + t('controller.spree.checkout.if you continue to receive this error then please contact us about th    is issue')
    redirect_to cart_path
  end
 
  rescue_from Spree::Calculator::CouldNotCalculateTax do |exception|
    flash[:error] = t('controller.spree.checkout.please verify your shipping address') + ' ' + t('controller.spree.checkout.if you continue to receive this error then please contact     us about this issue.')
    redirect_to checkout_path
  end

end