Make error message unique

Summary: We're currently getting errors for every duplicate key and can't easily unify them, so we're adding the additional information to a warning instead.

Reviewed By: mweststrate

Differential Revision: D30337821

fbshipit-source-id: db9dc44d7d3424de169bed9b4447b482e411eb19

desktop/flipper-plugin/src/data-source/__tests__/datasource-basics.node.tsx

@@ -116,7 +116,7 @@ test('throws on invalid keys', () => {
   }).toThrow(`Invalid key value: ''`);
   expect(() => {
     ds.append({id: 'cookie', title: 'test'});
-  }).toThrow(`Duplicate key: 'cookie'`);
+  }).toThrow(`Duplicate key`);
 });
 
 test('throws on update causing duplicate key', () => {