HBASE-8530 Refine error message from ExportSnapshot when there is leftover snapshot in target cluster (Ted Yu)



git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.94@1481581 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/0.94' into 0.94-gd
Merge remote branch 'origin/0.94-gd' into fb-YAHOO-317-0.94-gd