[macCreatorType] fix KeyError raised by old version of xattr module
[macCreatorType] fix KeyError raised by old version of xattr module

in version 0.6.4 (installed on OS X 10.10 python lib/extras) it raises
KeyError, whereas in the latest version available fom PyPI (v0.7.8) it
raises IOError.

Fixes issue in https://github.com/googlei18n/nototools/issues/113

fonttools/fonttools,googlefonts/fonttools