product access should check for broken symbolic links

[joefutrelle]

This appears to primarily be an issue with find_product_file where it assumes that all files returned by os.scandir exist. But os.scandir will list broken symbolic links.

os.scandir returns directory entries, it looks like is_file can be used to determine if a symbolic link is broken.

The desired behavior is to raise a KeyError if the product file is a broken symbolic link, just as if no existing file is found.

[joefutrelle]

Actually it looks like I'm just missing a final test for os.path.exists in some cases. The code assumes that any filename returned by os.scandir points to an existing file.