Add next/error

[nkzawa]

This PR enables to import default error page component with next/error.
It's intended to be used for displaying errors occurred on getInitialProps like #746 (comment), thanks to @dizlexik for sharing his workaround !

Fixes #746

[timneutkens]

Looks good!

[sedubois]

@nkzawa Is there any recommendation on how to display this error page, in case it's not triggered in getInitialProps?

For example with Apollo client:

• the render method might receive a data.error parameter in case of some generic issue
• or simply, we may issue a GraphQL query which doesn't give any result, in which case render will receive empty data. In this case I'd like to display the error page you provide with a status of 404.

[nkzawa]

@sedubois You mean to render the error page on custom server ? It would be something like this.

res.statusCode = 404
app.renderError(err, req, res, pathname, query)

But basically, I think you should use getInitialProps rather than fetching data on custom server.

[sedubois]

@nkzawa I don't mean custom server, but simply rendering the error page from within a component. Not from getInitialProps but instead from e.g componentWillMount.

  componentWillMount() {
    if (this.props.XXX === YYY) {
      // render error page
    }
  }

[nkzawa]

@sedubois I think you can just do that on render ?

render () {
  if (this.props.XXX === YYY) {
     return <ErrorPage statusCode={500} />
  }
  ...
}

[sedubois]

Thanks, that works. I was rather wondering how to render only that component. But that will do for now.