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 
</svg>
                <image {...expectSVG} />
              </svg>,
            )}
            <p {...expectHTML} />
          </foreignObject>
          <image {...expectSVG} />
        </svg>
        <p {...expectHTML} />
      </div>,
    );
  });
  it('should unwind namespaces on uncaught errors', () => {
    function BrokenRender() {
      throw new Error('Hello');
    }
    expect(() => {
      assertNamespacesMatch(
        <svg {...expectSVG}>
          <BrokenRender />
        </svg>,
      );
    }).toThrow('Hello');
    assertNamespacesMatch(<div {...expectHTML} />);
  });
  it('should unwind namespaces on caught errors', () => {
    function BrokenRender() {
      throw new Error('Hello');
    }
    class ErrorBoundary extends React.Component {