Hang occurs if the cache is not a directory

[klacansky]

Below example causes hangs when reading the dataset. Probably C++ needs to raise an exception for C++ code to work correctly too. I think when we rework LoadDataset to take cache_dir optional argument, we can then raise exception in the LoadDataset function.

import os
import OpenVisus as ov

with open('test.txt', 'w') as f:
	f.write('hello\n')

os.environ['VISUS_CACHE'] = 'test.txt'
dataset = ov.LoadDataset(f'https://klacansky.com/open-scivis-datasets/fuel/fuel.idx?cached=1')
dataset.read()