This commit annotates errors from GlyphSet.readGlyph() with the details
of the glyph that originated them (e.g. name, path to glif). This is
implemented with a loose backport of PEP678, to avoid adding a wrapper
error that would be less specific and would break API compatibility.
In addition, this commit adds a test to ensure that the new details are
present (specifically, in the case of parsing invalid XML).
70ca6dec9b