Fix OSError: No such file or directory

If you import product_details, in `__init__.py` it goes to build a
ProductDetails instance. If the json_dir doesn't exist, that fails with
an OSError.

That's problematic because then it becomes difficult in some situations
to run `./manage.py update_product_details`.

This fixes that so that it checks to see if the directory exists first.

Fixes #34 

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