fix(tableau-server-linux-backup): do not proceed when tsm raises error

[samanmasarat]

A couple of weeks ago, we saw some errors in our backup process. By drilling down, we found out that the tsm command was not working correctly (because of issues on the server side). So, instead of returning the expected output, it returned some error logs. And on the later step (when it cleans up the old backups), it was trying to clean up the path which didn't exist (and it can be risky as well since it may delete some critical files/folders).

TLDR If tsm command output has an error, exit and don't continue the script.