Add error placeholder for request errors

[huozhi]

Resolves: #34

If there're errors occurred during the data fetching in useData, it will return a error property and stops hanging. Then you can use the returned error to display it or do whatever you need there. e.g.

function Example() {
  const { data, error } = useData('data', fetcher)
  return (
    <div>
      {error && <ErrorPlaceholder error={error} />}
      {data && <div>My data: {data}</div>}
    </div>
  )
}

If there's error occurred in topstories fetching it will display the below error. You can try to tweaks the fetching url to simulate an error

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