**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.

it('should have been called even if the prop is not present', function() {
      var spy = jasmine.createSpy();
      Component = React.createClass({
        propTypes: {num: spy},
        render: function() {
          return <div />;
        },
      });
      var instance = <Component bla={5} />;
      instance = ReactTestUtils.renderIntoDocument(instance);
      expect(spy.calls.count()).toBe(1);
      expect(spy.calls.argsFor(0)[1]).toBe('num');
    });
    it('should have received the validator\'s return value', function() {
      spyOn(console, 'error');
      var spy = jasmine.createSpy().and.callFake(
        function(props, propName, componentName) {
          if (props[propName] !== 5) {
            return new Error('num must be 5!');
          }
        }
      );
      Component = React.createClass({
        propTypes: {num: spy},
        render: function() {
          return <div />;
        },
      });
      var instance = <Component num={6} />;
      instance = ReactTestUtils.renderIntoDocument(instance);