In FileBackend:

In FileBackend:
* Made secure() call doPrepare() for caller dummy proofing (especially those that don't check the status).
In FSFileBackend:
* Removed redundant wfMkdirParents() calls in FSFileBackend, we already have prepare() for this purpose. This also keeps it's behavior more consistent with the other backends.
* Made use of 'backend-fail-store' message in doStoreInternal().
Other:
* Updated unit tests and renamed $src => $source in some functions for consistency.
* Added some documentation comments and @since tags.