Exit code should be non-zero for inexistent user

[elifarley]

The command su-exec inexistent_user id -un erroneously reports root and has 0 as exit code.

[tianon]

Can confirm: 😢

bash-4.3# su-exec logstash id
uid=0(root) gid=0(root) groups=0(root),0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel),11(floppy),20(dialout),26(tape),27(video)

[ofek]

This was fixed by #26 which was released in https://github.com/ncopa/su-exec/releases/tag/v0.3