Add error-handler for getUserMedia

Hi there,

Due mostly to laziness, I haven't gotten around to playing with WebRTC from scratch. Stumbled across your demo, and decided it almost does what I need, except that it didn't work in modern browsers. This pull request gets it working with the latest Chrome, but Firefox is another story (looks like I need https://github.com/GoogleChrome/webrtc/blob/master/samples/web/js/adapter.js or similar to genericise function calls). I may try to work out what's going on there.

Further, would you mind adding a license to this code so that I can use it on my site? Given that it's currently unlicensed, I have no idea what I'm allowed to do with it.