RelayDefaultNetworkLayer, throw Error also on non-200 response.

src/network-layer/default/RelayDefaultNetworkLayer.js

@@ -163,7 +163,11 @@ function throwOnServerError(response: any): any  {
   if (response.status >= 200 && response.status < 300) {
     return response;
   } else {
-    throw response;
+    return response.text().then(text => {
+      const error = new Error(text);
+      (error: any).status = response.status;
+      throw error;
+    });
   }
 }
 

src/network-layer/default/__tests__/RelayDefaultNetworkLayer-test.js

@@ -36,6 +36,14 @@ describe('RelayDefaultNetworkLayer', () => {
     };
   }
 
+  function genFailureResponse(data) {
+    return {
+      json: () => Promise.resolve(data),
+      text: () => Promise.resolve(JSON.stringify(data)),
+      status: 500,
+    };
+  }
+
   beforeEach(() => {
     jest.resetModuleRegistry();
 
@@ -229,6 +237,31 @@ describe('RelayDefaultNetworkLayer', () => {
       expect(error.source).toEqual(response);
     });
 
+    it('handles server-side non-2xx  errors', () => {
+      const response = {
+        errors: [{
+          message: 'Something went completely wrong.'
+        }],
+      };
+
+      expect(fetch).not.toBeCalled();
+      networkLayer.sendMutation(request);
+      expect(fetch).toBeCalled();
+      const failureResponse = genFailureResponse(response);
+      expect(failureResponse.status >= 300 || failureResponse.status < 200).toBeTruthy();
+
+      fetch.mock.deferreds[0].resolve(failureResponse);
+      jest.runAllTimers();
+
+      expect(rejectCallback.mock.calls.length).toBe(1);
+      const error = rejectCallback.mock.calls[0][0];
+      expect(error instanceof Error).toBe(true);
+      expect(error.message).toEqual('{"errors":[{"message":"Something went ' +
+        'completely wrong."}]}');
+      expect(error.status).toEqual(failureResponse.status);
+      expect(error.source).toBe(undefined);
+    });
+
   });
 
   describe('sendQueries', () => {