Changes no-unused-expressions lint from warning to error

[amupitan]

Build crashes whenever JSX is not returned as proposed here: #2303.

The build crashes whenever JSX isn't returned, as opposed to just giving a warning.

To test:

Create a stateless component without a return clause.
For example:
import React from 'react';
export default function Logo(){
<div className="App-header">
<img src="./logo.svg" className="App-logo" alt="logo" />
<h2>Welcome to React</h2>
</div>
}

[gaearon]

I’m going to come back to this after we’re ready to cut over to 2.0 (with some other breaking changes). Thanks!

[kachkaev]

It'd be great if you considered adding allowTaggedTemplates: true if accepting this PR. See motivation in eslint/eslint#7632.

Without allowTaggedTemplates: true, things like this will cause an error, which is definitely not wanted:

import { injectGlobal } from 'styled-components';

injectGlobal`
  body {
    background: #f00,
  }
`;