"Generating fonts from Designspace failed: argument out of range" is not very helpful

"Generating fonts from Designspace failed: argument out of range" does not tell me WHAT argument was out of range, nor give me any idea where to go look. In fact, the problem data was not even in the Designspace file!

I had a wrong sign on the os2.usWinDescent, but Fontmake itself didn’t tell me that usWinDescent was the problem; only Pilcrow gave me the full detail (even though it got that info FROM Fontbakery): “invalid usWinDescent”.

@simoncozens can explain in more detail why Pilcrow gets this and Fontmake does not tell people more detail

## Observed behaviour

Error from Fontmake was generic and non-specific "argument out of range"

## Expected behaviour

Error message such as “invalid usWinDescent” would be good.

## Resources and exact process needed to replicate

This happened on a proprietary Google font, but if you reverse the sign on your usWinDescent like I did (long story!) then you will likely trigger the same problem.