Non-existent source layers in DS cause misleading error message

This line will blow up if the named layer for a source does not exist:

https://github.com/googlefonts/ufo2ft/blob/15f14d3c623728221feabd743cba41638483e5f8/Lib/ufo2ft/util.py#L47

fontmake will produce this error message:

> fontmake.errors.FontmakeError: In '..\aktiv-grotesk-vf\sources\test_v5_aktiv.designspace': Generating fonts from Designspace failed: '{126,100,0}'

which is a bit terse. Traceback:

```
Traceback (most recent call last):
File "c:\...\fontmake\lib\fontmake\font_project.py", line 1057, in run_from_designspace
self._run_from_designspace_interpolatable(
File "c:\...\fontmake\lib\fontmake\font_project.py", line 1147, in _run_from_designspace_interpolatable
self.build_variable_fonts(
File "c:\...\fontmake\lib\fontmake\font_project.py", line 378, in build_variable_fonts
fonts = ufo2ft.compileVariableTTFs(
File "c:\...\ufo2ft\lib\ufo2ft\__init__.py", line 588, in compileVariableTTFs
vfNameToBaseUfo = _compileNeededSources(
File "c:\...\ufo2ft\lib\ufo2ft\__init__.py", line 774, in _compileNeededSources
ttfDesignSpace = compileInterpolatableFunc(
File "c:\...\ufo2ft\lib\ufo2ft\__init__.py", line 378, in compileInterpolatableTTFsFromDS
for source, ttf in zip(result.sources, ttfs):
File "c:\...\ufo2ft\lib\ufo2ft\__init__.py", line 291, in compileInterpolatableTTFs
glyphSets = call_preprocessor(ufos, **kwargs)
File "c:\...\ufo2ft\lib\ufo2ft\__init__.py", line 63, in call_preprocessor
preProcessor = preProcessorClass(
File "c:\...\ufo2ft\lib\ufo2ft\preProcessor.py", line 280, in __init__
self.glyphSets = [
File "c:\...\ufo2ft\lib\ufo2ft\preProcessor.py", line 281, in
_GlyphSet.from_layer(
File "c:\...\ufo2ft\lib\ufo2ft\util.py", line 49, in from_layer
layer = font.layers[layerName]
File "C:\...\fontmake\.venv\lib\site-packages\ufoLib2\objects\layerSet.py", line 227, in __getitem__
layer_object = self._layers[name]
```

I wonder at which level of the stack we should catch this?