open_directory_store when directory does not exist

[Anniepoo]

We should check for directory existance when opening the store.

On a machine WITHOUT a /tmp/demo directory:

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?- open_directory_store('/tmp/demo', Store).
Store = <store_blob> .

?- open_write($Store, Builder).
ERROR: Unhandled exception: terminus_store_rust_error('No such file or directory (os error 2)')

EXPECTED: throw exception from open_directory_store/2
OBSERVED: lazily only flips when the store's written

[spl]

See #45.