FIX: handling non-zero exitcode in case of tar-warnings
- since tar V1.16
  - an exitcode of 0 - indicates no problems
  - an exitcode of 1 - indicates a warning
    warnings can be prevented from display by using the --warning option
    - http://www.gnu.org/software/tar/manual/html_section/warnings.html
  - an exitcode of 2 - indicates an error
Merge branch 'master' of https://github.com/SvenRoederer/backup-script