Based on Miguel Sousa's original PR and the following discussion:
https://github.com/googlei18n/cu2qu/pull/114
Instead of raising an error at the first incompatible glyph, we
let it continue (keeping the original contours unmodified when
that happens), and use logging to print error messages.
A new `IncompatibleFontsError` exception is raised at the end of
`fonts_to_quadratic` if any glyph has incompatible number or types
of segments. The exception instance has a `glyph_errors` attribute
(dict) which collects all the individual IncompatibleGlyphsError
keyed by glyph name.
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