bsutil: fix error msg for `--extra-config` parsing
bsutil: fix error msg for `--extra-config` parsing

dalehamel/minikube,dalehamel/minikube,dalehamel/minikube,kubernetes/minikube,warmchang/minikube,kubernetes/minikube,kubernetes/minikube,kubernetes/minikube,warmchang/minikube,kubernetes/minikube,kubernetes/minikube,warmchang/minikube,warmchang/minikube,warmchang/minikube,warmchang/minikube,warmchang/minikube,kubernetes/minikube