WSGI mini-app to create error output By default, this just uses the 'error_status', 'error_headers', and 'error_body' attributes to generate an output page. It can be overridden in a subclass to dynamically generate diagnostics, choose an appropriate message for the user's preferred language, etc. Note, however, that it's not recommended from a security perspective to spit out diagnostics to any old user; ideally, you should have to do something special to enable diagnostic output, which is why we don't include any here!