Config not exist

When the user clearly specifies a `--config` file, it would be helpful to warn when that file doesn't exist.
Otherwise he is left wondering why his options don't apply.

The reasoning here is that if I specifically point `rc` to a config file, I'm most likely making a mistake if that file doesn't exist.

This is based off `--rcconfig` PR in order to avoid merge conflicts -- so once the former is merged, only the last commit here needs to be applied.