Environment.Exit() is bad

[rasta-mouse]

Describe the bug

Maybe not fair to call this a bug, but Seatbelt calls Environment.Exit() here:
https://github.com/GhostPack/Seatbelt/blob/master/Seatbelt/Runtime.cs#L77

When Seatbelt is being run via (or integrated into) a tool/implant like Covenant's Grunt that runs the assembly within the implant's process (i.e. no fork & run) - it will cause the whole implant to exit when this exception is caught.

It would be nicer to orchestrate a way of gracefully returning so we can drop out of the assembly without explicitly calling this exit method.

To Reproduce

1. Start a Grunt.
2. Run Seatbelt with some remote enumeration options that will fail, e.g: Seatbelt -group=remote -computername=something.local -username=DOMAIN\fake -password=DoesNotMatter.
3. Watch the Grunt exit.

[leechristensen]

Thanks! Fixing in #36

[HarmJ0y]

Reviewed and landed into master. Thanks @rasta-mouse !

[rasta-mouse]

Thanks for the awesome turn around time guys ♥