**Do you want to request a _feature_ or report a _bug_?**
Enhancement

**What is the current behavior?**
Element type checker is not as informative as the other ones.

**What is the expected behavior?**
It would be nice to provide the actual type of the prop passed similarly to other type checkers. I.e., instead of 

> Invalid prop `Prop` supplied to `Component`, expected a single ReactElement.

say

> Invalid prop `Prop` of type `type` supplied to `Component`, expected a single ReactElement.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
Seeing this on 15.2.1 but I think it's also applicable to previous versions.

I know the general intention is to move away from prop types, but if there isn't a specific reason for the current behavior I can look into making this more pretty.

PropTypes.objectOf(PropTypes.number),
        [1, 2],
        'Invalid prop `testProp` of type `array` supplied to ' +
        '`testComponent`, expected an object.'
      );
      typeCheckFail(
        PropTypes.objectOf(PropTypes.number),
        123,
        'Invalid prop `testProp` of type `number` supplied to ' +
        '`testComponent`, expected an object.'
      );
      typeCheckFail(
        PropTypes.objectOf(PropTypes.number),
        'string',
        'Invalid prop `testProp` of type `string` supplied to ' +
        '`testComponent`, expected an object.'
      );
      typeCheckFail(
        PropTypes.objectOf(PropTypes.symbol),
        Symbol(),
        'Invalid prop `testProp` of type `symbol` supplied to ' +
        '`testComponent`, expected an object.'
      );
    });
    it('should not warn when passing an empty object', function() {
      typeCheckPass(PropTypes.objectOf(PropTypes.number), {});
    });
    it('should be implicitly optional and not warn without values', function() {
      typeCheckPass(PropTypes.objectOf(PropTypes.number), null);
      typeCheckPass(PropTypes.objectOf(PropTypes.number), undefined);