Add user-friendly error message when removing overlaps from quadratic UFO (unsupported)

I tried inputting a .glyphs file I'm working on and got this error:
```
INFO:fontmake.font_project:Building master UFOs and designspace from Glyphs source
INFO:glyphsLib:Parsing .glyphs file
INFO:glyphsLib:Casting parsed values
INFO:glyphsLib:Loading to UFOs
INFO:glyphsLib.util:Writing master_ufo/Cantarell-Light.ufo
INFO:glyphsLib.util:Writing master_ufo/Cantarell-Regular.ufo
INFO:glyphsLib.util:Writing master_ufo/Cantarell-SemiBold.ufo
Traceback (most recent call last):
File "/home/nikolaus/.local/bin/fontmake", line 11, in
sys.exit(main())
File "/home/nikolaus/.local/lib/python3.5/site-packages/fontmake/__main__.py", line 183, in main
project.run_from_glyphs(glyphs_path, **args)
File "/home/nikolaus/.local/lib/python3.5/site-packages/fontmake/font_project.py", line 376, in run_from_glyphs
glyphs_path, family_name, mti_source=mti_source)
File "/home/nikolaus/.local/lib/python3.5/site-packages/fontTools/misc/loggingTools.py", line 372, in wrapper
return func(*args, **kwds)
File "/home/nikolaus/.local/lib/python3.5/site-packages/fontmake/font_project.py", line 77, in build_master_ufos
family_name=family_name)
File "/home/nikolaus/.local/lib/python3.5/site-packages/glyphsLib/__init__.py", line 94, in build_masters
ufos, master_dir, designspace_instance_dir, instance_data)
File "/home/nikolaus/.local/lib/python3.5/site-packages/glyphsLib/interpolation.py", line 74, in build_designspace
write_ufo(font, master_dir)
File "/home/nikolaus/.local/lib/python3.5/site-packages/glyphsLib/util.py", line 41, in write_ufo
ufo.save(out_path)
File "/home/nikolaus/.local/lib/python3.5/site-packages/defcon/objects/font.py", line 749, in save
self.layers.save(writer, saveAs=saveAs, progressBar=progressBar)
File "/home/nikolaus/.local/lib/python3.5/site-packages/defcon/objects/layerSet.py", line 317, in save
layer.save(glyphSet, saveAs=saveAs, progressBar=progressBar)
File "/home/nikolaus/.local/lib/python3.5/site-packages/defcon/objects/layer.py", line 561, in save
self.saveGlyph(glyph, glyphSet, saveAs=saveAs)
File "/home/nikolaus/.local/lib/python3.5/site-packages/defcon/objects/layer.py", line 577, in saveGlyph
glyphSet.writeGlyph(glyph.name, glyph, glyph.drawPoints)
File "/home/nikolaus/.local/lib/python3.5/site-packages/ufoLib/glifLib.py", line 345, in writeGlyph
data = writeGlyphToString(glyphName, glyphObject, drawPointsFunc, formatVersion=formatVersion)
File "/home/nikolaus/.local/lib/python3.5/site-packages/ufoLib/glifLib.py", line 575, in writeGlyphToString
drawPointsFunc(pen)
File "/home/nikolaus/.local/lib/python3.5/site-packages/defcon/objects/glyph.py", line 464, in drawPoints
contour.drawPoints(pointPen)
File "/home/nikolaus/.local/lib/python3.5/site-packages/defcon/objects/contour.py", line 603, in drawPoints
pointPen.addPoint((point.x, point.y), segmentType=point.segmentType, smooth=point.smooth, name=point.name, identifier=point.identifier)
File "/home/nikolaus/.local/lib/python3.5/site-packages/ufoLib/glifLib.py", line 1485, in addPoint
raise GlifLibError("offcurve occurs before %s point." % segmentType)
ufoLib.glifLib.GlifLibError: offcurve occurs before line point.
```
I now know that I have to look into my semibold, but have no idea which glyph is the culprit. It would be great if fontmake would report this.