Instantiator: clearer error message when contour numbers of glyphs don't match

1. Get https://github.com/eellak/gsoc2018-cantarell/blob/master/01_GSoC%20FINAL/02_GLYPHS/Cantarell_20180814.glyphs
2. `fontmake -g Cantarell_20180814.glyphs -i -o ttf`

```
fontmake: Error: In 'Cantarell_20180814.glyphs' -> 'master_ufo/Cantarell.designspace': Interpolating instance 'Light_20180814' failed.: Failed to generate instance of glyph 'dieresiscomb.loclVIET': list index out of range. (Note: the most common cause for an error here is that the glyph outlines are not point-for-point compatible or have the same starting point or are in the same order in all masters.)
```

The glyph in question has its contours duplicated and overlaid on top. This may need a change in a different level of the stack.