Add error page when CSV data cannot be loaded

[stefanb]

During the recent github outage the CSV file couldn't be loaded and the page stayed empty.

[lukarenko]

Should we cache it via api.sledilnik.org? API already have option to cache CSV files, no need to add JSON interface for now. @MihaMarkic can tell us more...

https://api.sledilnik.org/api/stats?format=csv

[stefanb]

Caching somewhere is an option too, it is just a question how many new points of failure we want to introduce on the way.

[lukarenko]

API is also cached via CloudFlare and last GH outage it was not bad. Better than not have CSV at all.