prevent recursion error with cyclical components and provide descriptive error message

[anthrotype]

Fixes googlefonts/fontmake#1066

cc @emmamarichal

[anthrotype]

@emmamarichal with this, you should now get a more friendly error message than a RecursionError, telling you exactly how to find the cycle:

$ fontmake ../Gelasio/master_ufo/Gelasio.designspace -o variable
INFO:fontmake.font_project:Building variable fonts variable_ttf/Gelasio-VF.ttf
INFO:ufo2ft:Features are compatible across masters; building later
INFO:ufo2ft:Pre-processing glyphs
INFO:ufo2ft.filters:Running EraseOpenCornersFilter on Gelasio-Regular
fontmake: Error: In '../Gelasio/master_ufo/Gelasio.designspace': Generating fonts from Designspace failed: cyclical component reference: Ccedilla -> cedillacomb.case -> cedilla -> cedillacomb => cedilla

[emmamarichal]

Excellent! Thank you @anthrotype!