merge YARN-731 from branch-2. RPCUtil.unwrapAndThrowException should unwrap remote RuntimeExceptions. Contributed by Zhijie Shen.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.1-beta@1492002 13f79535-47bb-0310-9956-ffa450edef68