Add a warning for macOS versions w/ Focus Modes rather than DnD

[jasikpark]

Thank you for making a wonderful app that I only found out about in the last year!

It's been very useful to prevent pesky notifications from covering up any screen recordings or screen shares, though I finally looked up the issues for why Muzzle was misbehaving and found #98.

Would it be possible to add a notification to the Muzzle app that notifies you about the issue rather than it randomly failing? Hopefully, that could keep the number of duplicate issues down for you as well!

Thanks again for a great utility!