HBASE-7538 Improve snapshot related error and exception messages



git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/hbase-7290@1445851 13f79535-47bb-0310-9956-ffa450edef68
HBASE-7536 Add test that confirms that multiple concurrent snapshot requests are rejected



git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/hbase-7290@1445852 13f79535-47bb-0310-9956-ffa450edef68