Suggestion: show invalid / unavailable font-family in config in "Configuration errors" window

[erf]

If i write an invalid font-family in config it is shown in the log, but not in the "Configuration errors" window. I wish it was shown in the latter, so that we know we have typed an invalid / unavailable font, or you might be tricked into thinking you are using a font which is not there.

[mitchellh]

I want to add a generic notices system to the core that the UI's can listen for and show. I may change this issue to track that. But this is exactly the type of thing I want to use that for.

[erf]

Would it make sense to hook up to the macOS Notification Center?