fix #11096: import directory uniqueness exception handling

To review, scrutinize the stack trace in the description of http://trac.openmicroscopy.org.uk/ome/ticket/11096 and consider if this new `makeCheckedDirs` will help. (Also check that imports didn't break, etc.)

It might also be worth actually opening a session then attempting some slow simultaneous imports in case the bug can be successfully triggered. (I haven't yet managed it.)