Flash messages are not shown after error redirects

I have an oldish Spree 2.0 online shop running on Rails 3.2, that I'm trying to customize with cleaner error handling, using

config.exceptions_app = self.routes

in my config/application.rb.

For debugging purposes I have turned

config.consider_all_requests_local = false

in my config/environments/development.rb

in my routes.rb I have:

match '/404', to: 'errors#file_not_found'

in my errors_controller I have:

def file_not_found
  flash[:error] =  'The page you requested could not be found.'
  flash.keep
  redirect_to :back
rescue ActionController::RedirectBackError
  flash[:error] =  'The page you requested could not be found.'
  flash.keep
  redirect_to '/'
end

I have tried several different ways of doing this, including the ones described in examples at http://ift.tt/1q8wGHM but always the result is, the flash message works if I specifically go to localhost:3000/404, but does not work if I go to some nonexistent URL, such as localhost:3000/asdfasdf.

In app/views/spree/layouts I have spree_application.html.erb, where I have

<%= flash_messages %>

which in turn is defined in/spree_core-2.0.10/app/helpers/spree/base_helper.rb

def flash_messages(opts = {})
  opts[:ignore_types] = [:commerce_tracking].concat(Array(opts[:ignore_types]) || [])

  flash.each do |msg_type, text|
    unless opts[:ignore_types].include?(msg_type)
      concat(content_tag :div, text, class: "flash #{msg_type}")
    end
  end
  nil
end