[HBASE-9930] Fix NPE in processListOfPuts.

Author: manukranthk

Summary: We were returning null in one of the locations when we failed to get a client object. We should be instead throwing an IOException in that case.

Test Plan: Run TestMultiParllelPut

Reviewers: gauravm, fan

Reviewed By: gauravm

CC: hbase-eng@

Differential Revision: https://phabricator.fb.com/D1205626

Task ID: 3864263

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1576885 13f79535-47bb-0310-9956-ffa450edef68