[VF] Warn users if no instances exist in fvar

In [RobotoVF](https://github.com/TypeNetwork/Roboto), the font [Roboto-VF.ttf](https://github.com/TypeNetwork/Roboto/blob/master/fonts/Roboto-VF.ttf) contains no instances in the fvar table.

```
>>> from fontTools.ttLib import TTFont
>>> f = TTFont('/Users/marc/Downloads/Roboto-VF.ttf')
>>> f['fvar'].instances
[]
```

fvar instances are used in GFR to generate css classes.

cc @laerm0