title: Karma and React, have warnings to cause errors
body: <p>I am using <a href="http://karma-runner.github.io/0.8/index.html">Karma</a> with <a href="http://mochajs.org/">mocha</a> to test my <a href="https://facebook.github.io/react/">React</a> components. I have some warnings displayed when the PropTypes are not matched. However it would be really interesting to have these warnings to cause an actual error, as to track down the test and fix it. </p>
  
  <p>Do you know how this could be achieved?</p>