Do not use os.Exit packages from `pkg`

[davidcassany]

Luet version:

luet version 0.22.0-g7f7e1418c1316149c5424bacaf7993f4daaa3e99 2021-12-27 10:49:33 UTC

CPU architecture, OS, and Version:

x86_64

Describe the bug

When using Luet as a library the use of os.Exit in several places limits the possibility of proper error handling. In fact I'd strictly limit the use the os.Exit to very limited and known places and not inside any exposed code potentially used as a library. In rancher-sandbox/elemental-cli this is causing issues to properly handle errors caused by luet.

To Reproduce

Run something like elemental-cli install /dev/sda --docker-image quay.io/costoolkit/releases-green:cos-system-0.7.11-18 from rancher/elemental-cli#34 and the cosign plugin will fail causing the program to exit without unmounting any of the images and partitions that were mounted during the installation. In fact non of the deferred functions will be executed.

Expected behavior

If there is an error on any of the exported luet packages this is not causing an os.Exit, at least a panic, so the code managing the library can decide to continue with the panic execution or eventually recover.

Logs

Additional context