Fix return status of summary within impala shell

Follow up fix to IMPALA-1153. Ensure that the correct
CmdStatus is returned by the summary command. (ERROR for
invalid queries and SUCCESS even if summary is not available.)

Change-Id: Icf67164dc82f202ec15071541f6ed3b26e3ad7fb
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4089
Reviewed-by: Abdullah Yousufi <abdullah.yousufi@cloudera.com>
Tested-by: jenkins