Retries

It would be more robust if the javascript code on the front-end part (re)tried again after an `XMLHttpRequest()` failure.

That'd be useful in particuler when running Django's built-in webserver on `http://localhost:8000` (using `manage.py runserver`) because when in development mode (`DEBUG=1`) Django's builtin webserver reloads python source files that change by restarting the webserver and the short unavailability of the webserver makes the progressbar stop (I'm using the default refresh rate of 500ms).

Thank you.