Add error reporting for frontend

[ribalba]

Currently there is no way for us the find out if there are errors in the frontend on metrics.green-coding.io. I would want some reporting that tells us if there is JS error.

https://sentry.io/welcome/ for example

[ArneTR]

At the moment we have something though, which is UptimeRobot.

But I agree. JS Tracking would be nicer. +1 for this.