Dragonfly patch to fall back to a default image when the original is not found.

gistfile1.rb

if Rails.env.development?  
  module Dragonfly
    module DataStorage
      
      class FileDataStore
        def retrieve(relative_path)
          validate_uid!(relative_path)
          path = absolute(relative_path)
          pathname = Pathname.new(path)
          if !pathname.exist?
            pathname = Pathname.new('/path/to/default-image.jpg')
          end
          raise DataNotFound, "couldn't find file #{ENV['RAILS_ROOT']}" unless pathname.exist?
          [
            pathname,
            (store_meta ? retrieve_meta_data(path) : {})
          ]
        end
      end
    end
  end    
end