Improve error message about missing credentials

[Programie]

As of now, if the config.json does not exist yet or has the wrong file permissions, ionosctl simply shows Error: username, password or token incorrect which might be a bit misleading if you are 100% sure the credentials are correct (especially if the config.json is there and it's just a permissions problem).

Maybe showing a warning in case the permissions are not correct could lead the user into the right direction. As an example, kubectl is also showing a warning if the ~/.kube/config file has too open file permissions. Same happens for SSH if the private key file is also readable by others.