RFE: Add pull secret pre-flight safety check & abort msg

Koffer failures may fail for reasons which appear ambiguous when provided with a corrupted json registry pull secret.

A simple pre-flight test such as the following would enable intuitive abort reason messaging:
```sh
jq . /root/.docker/config.json || exit 1
```

This would alleviate confusion around the most common koffer failure symptoms and enable the user to resolve on their own.