Stop base_url with bad API

With wrong api string the `base_url` return the `url` function from base. This fix will rise error if API is unknown.

The cran version does this with the example  `baseURL <- base_url("statfi", "v1", "fi")` from pxweb.md. It is fixed in the github for "statfi", but this will raise error with all non-valid api strings.