subclass of FeatureLibError, only raised if IOError.errno == ENOENT (i.e. FileNotFoundError) https://github.com/googlei18n/fontmake/issues/157#issuecomment-367380471 Will be useful in ufo2ft to detect when an included feature file doesn't exist and print a nicer error message explaining that includes must be relative to the UFO itself, and not relative to the embedded features.fea file. https://github.com/unified-font-object/ufo-spec/issues/55