Revisit CLI and error message copy

[rossf7]

Follow up to https://github.com/thegreenwebfoundation/grid-intensity-go/pull/49/files#r935719541

We should revisit the copy we're using in the CLI and error messages for consistency and best practices.

We could also investigate using an errors library like juju. https://github.com/juju/errors

This is to ensure we have good stack traces for debugging. As we add more functionality and providers this becomes more important and will make it easier for external contributors to work on the codebase.