Handle run_lxc_download_script failure

[Jip-Hop]

When run_lxc_download_script fails, jlmkr still returns 0 exit code.

root@ubuntu:~/jailmaker# ./jlmkr.py images
ERROR: Missing required tool: wget
root@ubuntu:~/jailmaker# echo $?
0

When run_lxc_download_script fails during jail creation, the jail is not cleaned up.

[Jip-Hop]

jlmkr create --distro=nonexistent test
USE THIS SCRIPT AT YOUR OWN RISK!
IT COMES WITHOUT WARRANTY AND IS NOT SUPPORTED BY IXSYSTEMS.
Creating jail test with default config.
Overriding distro config value with nonexistent.
Overriding systemd_nspawn_user_args config value with [].
Downloading the image index
ERROR: Couldn't find a matching image
Aborting...

Jail is created but the rootfs directory remains empty. Exit code is 1 in this case, but the failed jail is not cleaned up.

[Jip-Hop]

Awaiting lxc/lxc#4462 to decide how to handle this case:

root@ubuntu:~/jailmaker# ./jlmkr.py images
ERROR: Missing required tool: wget
root@ubuntu:~/jailmaker# echo $?
0

But jlmkr create --distro=nonexistent test now triggers a cleanup (develop branch).