use G's wording for error message

lib/read-metadata.js

@@ -142,7 +142,7 @@ async function readMetaData(path, {
     // file, then fail hard and let the user know that they need to pick one or the
     // other
     if (hasNowJson) {
-      const e = new Error('Refusing to proceed with multiple Now configurations (`now.json` and `package.json`). Please pick only one, then try again!')
+      const e = new Error('You have a `now` configuration field inside `package.json`, but configuration is also present in `now.json`! Please ensure there\'s a single source of configuration by removing one')
       e.userError = true
       throw e
     } else {

test/index.js

@@ -196,5 +196,6 @@ test('throws when both `now.json` and `package.json:now` exist', async t => {
     e = err
   }
   t.is(e.name, 'Error')
-  t.pass(/refusing to proceed/i.test(e.message))
+  t.is(e.userError, true)
+  t.pass(/please ensure there\'s a single source of configuration/i.test(e.message))
 })