OAK-1530: TarMK thread logs warning

Catch the http://bugs.java.com/bugdatabase/view_bug.do?bug_id=6539707
exceptions, and log a milder warning for those cases

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1579024 13f79535-47bb-0310-9956-ffa450edef68