cronjob should not always exit with a zero exit code

should exit non-zero in case of errors during the duplicity job so that e. g. cron monitors are alarmed if something went wrong.

remove obsolete conjunction of bash commands as we use "set -e" anyways.

"set -e" and "&&" do no play well together and result in dangerous
situation where the script would exit with code "0" although the
pre-command failed.
Merge pull request #11 from Jimdo/fix_exit_code

cronjob should not always exit with a zero exit code