fail hard when both a `now.json` and `package.json` file with "now" are present

As discussed in the GH issue.

lib/read-metadata.js

@@ -125,9 +125,11 @@ module.exports = async function getMetadata(path, {
     name = deploymentName
   }
 
-  // merge in `package.now` if present, but have `now.json` take precedence
+  // if the project has both a `now.json` and `now` Object in the `package.json`
+  // file, then fail hard and let the user know that they need to pick one or the
+  // other
   if (pkg.now) {
-    nowConfig = extend({}, pkg.now, nowConfig)
+    throw new Error('Refusing to proceed with multiple Now configurations (`now.json` and `package.json`). Please pick only one, then try again!')
   }
 
   return {