DERBY-6503: ClassCastException when network server cannot restrict file permissions

Make sure the underlying IOException is exposed if the network server
fails to restrict file permissions. The original exception used to be
shadowed by a ClassCastException.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1580789 13f79535-47bb-0310-9956-ffa450edef68