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.)
Could the object have been grabbed by someone else at this point?