Improved network-drive-resilience in index directory creation

In a product, the removed Files.exists(p) && !Files.isDirectory(p) code
threw an exception because the examined path was not guaranteed to be in
a coherent state at the time of examination but Files.createDirectories
would still take care of throwing any exceptions if there were any
problems in creating the directory.

The worst thing was that this exception was hidden and not even logged.
Luckily a problem case forced the find.

refs #7845

Change-Id: Id44c5118ad97fbb83dcec87f77717185d3e402a1