Changes to shouldContainText and shouldNotContainText messaging

shouldNotContainText error message was same as shouldContainText. Modified to accurately reflect the error condition.

Also, for both, now show the text found in the element as well as the expected text. I find this useful, and have created a customization in all my projects to handle this. Finally hoping to have it in core.

Once you've had a chance to review, let me know if you'd like anything changed or enhanced.