[HBASE-2856] [0.89-fb] [trunk version] Clean up the scannerReadPoing if there is an exception.

Author: aaiyer

Summary:
Clean up the scannerReadPoing if there is an exception.
See also https://phabricator.fb.com/D1037580

Test Plan: push to shadow and test

Reviewers: liyintang

Reviewed By: liyintang

CC: hbase-eng@

Differential Revision: https://phabricator.fb.com/D1037582

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1538243 13f79535-47bb-0310-9956-ffa450edef68