Missing configuration file behaves like an empty configuration file

I'm using, ```hs withConfigurableYamlConfig (xdgYamlConfigFile "...") ... ``` It seems, when no file exist at that location, processing proceeds with `{}`. For example, if my parser requires a keys, I get the `Missing config value: <the key>` error. And if my parser is all optional keys, it just silently succeeds. I think this should be an error that indicates the file was is missing, rather than silently using an empty value. It took me a long time to figure out what was happening.