Fix one of the TestCheckpoint test cases broken by merge.
The exception message in this test case is now different since
HDFS-2011 caused the storage directory to become invalidated at
roll-time instead of at checkpoint time.
git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-1073@1143527 13f79535-47bb-0310-9956-ffa450edef68