I feel like this is the wrong strategy: that environment variable is obviously expressing user intent.  There is no way you, say, accidentally used that for something else that I can imagine.

So, I think when it is present but not valid we should do something more aggressive than just ignoring it.  I think a fatal error is appropriate: terminate, highlight the problem, and let the user either unset the value, or fix it, before running again.

Use the default iff there is no pre-existing value.  (eg: `url = ENV['RAZOR_API_URL'] or default_url`, then demand that URL be valid.)