preinst: use "set -e" in the script body instead of in the shebang.

Detected by Lintian's maintainer-script-without-set-e check.

debian/backupninja.preinst

-#!/bin/sh -e
+#!/bin/sh
+set -e
 
 # Remove the erroneous /etc/logrotate.d/backupninja directory if it exists