IMPALA-1153: Avoid reprinting shell's welcome message after exceptions

Restructured how the cmd control flow executes commands in postcmd,
removing the hack to make non-interactive mode work. Now there are
values to represent the different command execution statuses.

Change-Id: I149b65d8a64d63a978fed284f0ad0da95833149c
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3850
Reviewed-by: Abdullah Yousufi <abdullah.yousufi@cloudera.com>
Tested-by: jenkins