KeyError exception during cache clear.

[wrkyle]

I run this stemmer in one of my APIs 24/7. Very rarely I see a KeyError exception coming from _clear_cache() in basestemmer.py, line 339. Unless this is a symptom of an underlying problem, catching the KeyError in _clear_cache would solve the problem.

I'm wondering how this could happen. Possibly a data race in a multiprocessing environment? I'd have to examine the environment in which I'm using this code.