https://codesandbox.io/s/40j50607y0

Rendering a component like:

```js
const Foo = () => <React.Fragment />
```

Throws the error:

> Foo(...): Nothing was returned from render. This usually means a return statement is missing. Or, to render nothing, return null.

The error is confusing because clearly `Foo` _is_ returning a value. I don't know if we want to support rendering empty fragments, but in any case this error is confusing.

If this isn't allowed it should explicitly call out that a component cannot return a fragment with no children.

cc @gaearon 
state = {error: null};
      componentDidCatch(error) {
        this.setState({error});
      }
      render() {
        if (this.state.error) {
          return <p {...expectHTML} />;
        }
        return this.props.children;
      }
    }
    assertNamespacesMatch(
      <svg {...expectSVG}>
        <foreignObject {...expectSVG}>
          <ErrorBoundary>
            <math {...expectMath}>
              <BrokenRender />
            </math>
          </ErrorBoundary>
        </foreignObject>
        <image {...expectSVG} />
      </svg>,
    );
    assertNamespacesMatch(<div {...expectHTML} />);
  });
  it('should unwind namespaces on caught errors in a portal', () => {
    function BrokenRender() {
      throw new Error('Hello');
    }
    class ErrorBoundary extends React.Component {