I just discovered this not too long ago...
I searched for something like this before, but nobody was talking about it so I'm posting it here as it looks like nobody knows...! I hope you'll like it.
- Open the font with the Font Book app
- Wait for the validation error message
- Menu: "File" > "Save Report..."
The report is in fact a simple ".plist" that can be opened in text mode or with Xcode.
It can help figuring out why the validation didn't pass but it is complimentary with `ftxvalidator` that is a lot more verbose.