only throw IOExc if merge really hit an exception

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1595239 13f79535-47bb-0310-9956-ffa450edef68
More cleanup, sorry for the bug I introduced! :(

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1595247 13f79535-47bb-0310-9956-ffa450edef68