Merge r1590941 from trunk, HDFS-2882. DN continues to start up, even if block pool fails to initialize (Contributed by Vinayakumar B)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.4@1590943 13f79535-47bb-0310-9956-ffa450edef68