**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.

var name = Thing.name || '<<anonymous>>';
      typeCheckFail(
        PropTypes.arrayOf(PropTypes.instanceOf(Thing)),
        [new Thing(), 'xyz'],
        'Invalid prop `testProp[1]` of type `String` supplied to ' +
        '`testComponent`, expected instance of `' + name + '`.'
      );
    });
    it('should warn when passed something other than an array', function() {
      typeCheckFail(
        PropTypes.arrayOf(PropTypes.number),
        {'0': 'maybe-array', length: 1},
        'Invalid prop `testProp` of type `object` supplied to ' +
        '`testComponent`, expected an array.'
      );
      typeCheckFail(
        PropTypes.arrayOf(PropTypes.number),
        123,
        'Invalid prop `testProp` of type `number` supplied to ' +
        '`testComponent`, expected an array.'
      );
      typeCheckFail(
        PropTypes.arrayOf(PropTypes.number),
        'string',
        'Invalid prop `testProp` of type `string` supplied to ' +
        '`testComponent`, expected an array.'
      );
    });
    it('should not warn when passing an empty array', function() {