🐛 correct error and exit status handling

[jeftadlvw]

Currently, command handling functions don't return an error, and no actual errors are handled at the root command. Thus, git-nest returns an exit status of 0, which is false behaviour when errors do occur.

Cobra has this functionality built-in:

• Command.Run should return error spf13/cobra#67
• Docs needed: how best to handle errors in Run spf13/cobra#914

I see no reason to not utilize RunE instead of Run.