Closes #139 - detect access to translated field on untranslated instance and:
-> raise an AttributeError, which is the correct semantic for a descriptor
-> provide a more helpful exception message
-> retain compatibility with code that may already catch a DoesNotExist exception. We should deprecate that though
Merge pull request #139 from spectras/notranslation_warn_139

Detect access to translated field on untranslated instance and raise an AttributeError