Restore default error page templates

In 21551fa95bb1cfc50edf82231b63ebda606fbcfc we forced all requests in
production to appear remote. This prevents rails from rendering
backtraces when unhandled exceptions are raised.

In alphagov/puppet@042cd87f9e617f56bd96d1fd8d900aa7d8f7db7b we
introduced an option enable/disable intercept_errors which defaults to
false, and whitehall leaves that as false.

The aim of these changes has been to allow error page content to be
shown to end users. In particular to display the custom 403 error
message generated by GDS::SSO when the user is authenticated by doesn't
have permission to the app.

So, since the error page content from the rails app is now shown to end
users, we need to restore these page templates.