Log a warning on DocumentNotFound

Log a warning when a `Mongoid::Errors::DocumentNotFound` error is raised
 in a controller. These errors were being silently swallowed by the
 application.

The error occurs when an attempt is made to load a document that doesn't
 exist, which can happen when the overnight DB replication from
 prod->integration fails. It will also happen if the edition ID is
 manipulated in the URL (e.g. in URLs of the form
 `/editions/[edition-id]`), so it's not _necessarily_ an error when it
 happens.

app/controllers/application_controller.rb

@@ -19,7 +19,8 @@ def setup_view_paths_for(publication)
     prepend_view_path template_folder_for(publication)
   end
 
-  def record_not_found
+  def record_not_found(exception)
+    Rails.logger.warn "Error loading document: #{exception.message}"
     render body: { 'raw': "404 Not Found" }, status: :not_found
   end