The ui doesn't let users know if the backend is working

[anubisg1]

The UI (cli and web ui) only read static data from parquet folder, that makes it impossible for a user to know if the backend system is actually working as expected.

The poller could be completely dead and the only way to know something is wrong is to look at timestamp of the data being visualized.

The web ui at least should report if the backend is no longer working.