throw an error if the API token is undefined

[zeke]

I was doing this:

import Replicate from "replicate";

const replicate = new Replicate({
  auth: process.env.REPLICATE_API_TOKEN,
});

But REPLICATE_API_TOKEN wasn't actually set in the env. So I was getting "Not found" errors. We should check for this and blow up with an error ahead of time, before making the request.