Better error messages for missing config vars

[kernel-panic96]

Hello, it would be nice if the error for when there is a missing variable expansion states that it is missing instead of "empty default". Example below.

go version - 1.21.2

uber-go/config version - 1.4.0

sidecar:
  global-tags:
    - "pod=$MY_POD_NAME"
    - "namespace=$MY_NAMESPACE"

main: go

func main() {
	cfgProvider, err := config.NewYAML(config.Source(strings.NewReader("sidecar: { log-level: INFO, global-tags: [pod=$POD, namespace=$NAMESPACE]}")), config.Expand(os.LookupEnv))
	if err != nil {
		zap.L().Fatal("could not read yaml config", zap.Error(err))
	}
	var cfg AppConfig
	if err := cfgProvider.Get("sidecar").Populate(&cfg); err != nil {
		zap.L().Fatal("could not read app config", zap.Error(err))
	}
}

output:

go run main.go # no exported env vars
{"level":"fatal","message":"could not read yaml config","error":"couldn't expand environment: default is empty for \"POD\" (use \"\" for empty string)"}

It would be clearer if the error was - "missing expansion variable {VAR_NAME}" instead of "default is empty".