Rather minor issue, but the fix should be simple enough so I thought that I'd file this anyway:

When using `react-addons-testutils`, sometimes I write simple errors such as 

```
TestUtils.scryRenderedDOMComponentsWithClass("foo")

// expected:
TestUtils.scryRenderedDOMComponentsWithClass(reactComponentInstance, "foo")

```

This throws a stack trace of ~80 lines with a type error of `TypeError: Cannot read property 'split' of undefined`. Fixing this is trivial (and the first line of the stack trace points to the offending line as expected), but the error itself isn't really obvious. 

Some others such as `scryRenderedDOMComponentsWithTag` don't throw until `findAllInRenderedTree` is reached, which throws a bit more helpful error of `Error: findAllInRenderedTree(...): instance must be a composite component(…)`. 

Would it make sense to check that the arguments passed are valid and throw immediately if they aren't, so the user would end up with errors along the lines of `scryRenderedDOMComponentsWithClass: first argument must be an instance of ReactComponent`?

);
    if (all.length !== 1) {
      throw new Error(
        'Did not find exactly one match (found: ' + all.length + ') ' +
        'for componentType:' + componentType
      );
    }
    return all[0];
  },
  /**
   * Pass a mocked component module to this method to augment it with
   * useful methods that allow it to be used as a dummy React component.
   * Instead of rendering as usual, the component will become a simple
   * <div> containing any provided children.
   *
   * @param {object} module the mock function object exported from a
   *                        module that defines the component to be mocked
   * @param {?string} mockTagName optional dummy root tag name to return
   *                              from render method (overrides
   *                              module.mockTagName if provided)
   * @return {object} the ReactTestUtils object (for chaining)
   */
  mockComponent: function(module, mockTagName) {
    mockTagName = mockTagName || module.mockTagName || 'div';
    module.prototype.render.mockImplementation(function() {
      return React.createElement(
        mockTagName,
        null,
        this.props.children
      );