**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.

typeCheckFail(
        PropTypes.instanceOf(RegExp),
        {},
        'Invalid prop `testProp` of type `Object` supplied to ' +
        '`testComponent`, expected instance of `' + regExpName + '`.'
      );
      typeCheckFail(
        PropTypes.instanceOf(Person),
        new Cat(),
        'Invalid prop `testProp` of type `Cat` supplied to ' +
        '`testComponent`, expected instance of `' + personName + '`.'
      );
      typeCheckFail(
        PropTypes.instanceOf(Person),
        Object.create(null),
        'Invalid prop `testProp` of type `<<anonymous>>` supplied to ' +
        '`testComponent`, expected instance of `' + personName + '`.'
      );
    });
    it('should not warn for valid values', function() {
      function Person() {}
      function Engineer() {}
      Engineer.prototype = new Person();
      typeCheckPass(PropTypes.instanceOf(Person), new Person());
      typeCheckPass(PropTypes.instanceOf(Person), new Engineer());
      typeCheckPass(PropTypes.instanceOf(Date), new Date());
      typeCheckPass(PropTypes.instanceOf(RegExp), /please/);
    });
    it('should be implicitly optional and not warn without values', function() {
      typeCheckPass(PropTypes.instanceOf(String), null);