Warn about letsencrypt certificates errors

[nirgal]

Connections from android to a turn server with letsencrypt certificates fail with error:
TLSv1.2 Record Layer: Alert (Level: Fatal, Description: Unknown CA)

This is the same issue than jitsi/jitsi-meet#5589

This remark will hopefully save some trouble to other people.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
• Pull request includes a sign off
• Code style is correct (run the linters)

[nirgal]

Too long to make a compliant PR. Closing.