Add glyphname when crashing in makeSourceGlyphsCompatible

[LettError]

Would be helpful to catch this traceback and add which glyph is causing trouble.

File "/Users/erik/Library/Application Support/RoboFont/plugins/Batch.roboFontExt/lib/batchGenerators/variableFontsGenerator/init.py", line 158, in makeSourceGlyphsCompatible
AttributeError: 'NoneType' object has no attribute 'extractGlyph'