IMPALA-919: Clarify error message for bad explain levels

Change-Id: I1c446ec75578343446d9f5c989467c294cba0ca7
(cherry picked from commit 00e6e0cb6b54b2056a7014323ffec21a94538ce1)
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5525
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: jenkins