[15] 8220816: Files.createDirectory should make it more obvious that it fails when the directory already exists

[Brian Burkhalter]

Please review this minor change [1] to address [2]. If it looks worthwhile to address then a CSR will be filed.

Thanks,

Brian

[1] http://cr.openjdk.java.net/~bpb/8220816/webrev.00/
[2] https://bugs.openjdk.java.net/browse/JDK-8220816