HBASE-7469. [REST] Share a HBaseAdmin instance


git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.94@1427227 13f79535-47bb-0310-9956-ffa450edef68
HBASE-7464. [REST] Sending HTML for errors is unhelpful


git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.94@1427568 13f79535-47bb-0310-9956-ffa450edef68