LUCENE-4738: simplify DirectoryReader.indexExists; fix IndexWriter with CREATE to succeed on a corrupted index; add random IOExceptions to MockDirectoryWrapper.openInput/createOutput

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1466707 13f79535-47bb-0310-9956-ffa450edef68