title: GraphQL / Apollo - Can't get NetworkError
body: <p>I'm unable to get any network error with my Apollo Client, only GraphQL errors. </p>
  
  <p>I have the client set up like so:</p>
  
  <pre><code>    const errorLink = onError(({ graphQLErrors, networkError }) =&gt; {
    if (graphQLErrors)
      graphQLErrors.map(({ message, locations, path }) =&gt;
        console.log(`[GraphQL error]: Message: ${message}, Location: ${locations}, Path: ${path}`)
      );
    if (networkError) console.log(`[Network error]: ${networkError}`);
  });
  
    const client = new ApolloClient({
      link: ApolloLink.from([
        errorLink,
        new HttpLink({
          uri: 'http://localhost:4000/graphql'
        }),
      ]),
    });
  </code></pre>
  
  <p>On the server i'm making a request which i parse like this: </p>
  
  <pre><code> .then(res =&gt; {
    if (res.ok) {
      return { blah: res.json(), count };
    }
    throw new Error();
  })
  .catch(error =&gt; {
    throw new AuthenticationError('Must Authenticate');
  });
  </code></pre>
  
  <p>The AuthenicationError just bubbles up to the Client as a GraphQL Error (<code>[GraphQL error]: Message: Must Authenticate,</code>, i basically just want to be able to get the HTTP Status Code from the server request, on the client. </p>
  
  <p>Thanks</p>