Return some meaningful error code. Unfortunately the more meaningful
option (retrieving process exit code) can lead to false errors
(described in comment), but at least report exit code of tar2qfile.
Do not send 'which' command output to stdout, as it will mess real
backup data.
This fixes regression introduced by this commit:
commit dad5bfbd18
Author: HW42 <hw42@ipsumj.de>
Date: Thu Feb 5 03:14:41 2015 +0100
remove 'bashisms' or explicit use bash
