EZP-21412: skip unstored cache (race condition)
EZP-21412: todo
EZP-21412: unified usage of _storeContents()

Following the additions to dfs/oracle in https://github.com/ezsystems/ezoracle/pull/15,
eZDFSFileHandler was made a bit smarter when storing cache.

It will abort generation if an error occured, and ignore the case
where cache couldn't be stored for a known reason.