**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

/**
   * Mounted components have a DOM node representation and are capable of
   * receiving new props.
   */
  | 'MOUNTED'
  /**
   * Unmounted components are inactive and cannot receive new props.
   */
  | 'UNMOUNTED';
function getLifeCycleState(instance): ComponentLifeCycle {
  return instance.updater.isMounted(instance) ? 'MOUNTED' : 'UNMOUNTED';
}
/**
 * TODO: We should make any setState calls fail in
 * `getInitialState` and `componentWillMount`. They will usually fail
 * anyways because `this._renderedComponent` is empty, however, if a component
 * is *reused*, then that won't be the case and things will appear to work in
 * some cases. Better to just block all updates in initialization.
 */
describe('ReactComponentLifeCycle', () => {
  beforeEach(() => {
    jest.resetModules();
    React = require('react');
    ReactDOM = require('react-dom');
    ReactTestUtils = require('react-dom/test-utils');
    PropTypes = require('prop-types');
  });
  it('should not reuse an instance when it has been unmounted', () => {
    const container = document.createElement('div');
    class StatefulComponent extends React.Component {