**Do you want to request a *feature* or report a *bug*?**

bug

**What is the current behavior?**

The warning for a failure to initialize state when using `getDerivedStateFromProps` is not as clear as it could be.

This component produces the warning:
> MyComponent: Did not properly initialize state during construction. Expected state to be an object, but it was undefined.`

```js
class MyComponent extends React.Component {
  static getDerivedStateFromProps(nextProps, prevState) {
    return null;
  }
  render() {
    return <span>Foo</span>
  }
}
```

The warning can be resolved by adding `state = {}` either in the class definition or the constructor, or by not using `getDerivedStateFromProps`.

See https://github.com/reactjs/reactjs.org/issues/796 for more detail on why this warning message can be counter-intuitive.

Since the [warning is specific](https://github.com/facebook/react/blob/b548b3cd640dbd515f5d67dafc0216bb7ee0d796/packages/react-reconciler/src/ReactFiberClassComponent.js#L509-L510) to the use of `getDerivedStateFromProps` and not triggered by any other state or lifecycle methods, and state initialization isn't required for other state methods like `this.setState`, it would make sense to me to mention `getDerivedStateFromProps` explicitly in the warning.

Here's a proposal for a new warning message:
> MyComponent: Component state must be initialized when using getDerivedStateFromProps. Expected state to be an object, but it was undefined.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**

React >= 16.3.0

constructor(props, context) {
        super(props, context);
        this.state = {tag: context.tag, className: this.context.className};
      }
      render() {
        const Tag = this.state.tag;
        return <Tag className={this.state.className} />;
      }
    }
    Foo.contextTypes = {
      tag: PropTypes.string,
      className: PropTypes.string,
    };
    class Outer extends React.Component {
      getChildContext() {
        return {tag: 'span', className: 'foo'};
      }
      render() {
        return <Foo />;
      }
    }
    Outer.childContextTypes = {
      tag: PropTypes.string,
      className: PropTypes.string,
    };
    test(<Outer />, 'SPAN', 'foo');
  });
  it('renders only once when setting state in componentWillMount', () => {
    let renderCount = 0;
    class Foo extends React.Component {