Propagate transport fail state (e.g. 500)

When the state 'TRANSPORT_FAILED' is raised it is propagated to the listener. This would allow to subscribe to a onError callback in javascript for example:
https://github.com/ging/licode/blob/master/erizo_controller/erizoJS/erizoJSController.js
```
wrtc.init( function (newStatus){
          var localSdp, answer;
          log.info("webrtc Addon status" + newStatus );
          if (newStatus === 102 && !sdpDelivered) {
            localSdp = wrtc.getLocalSdp();
            answer = getRoap(localSdp, roap);
            callback('callback', answer);
            sdpDelivered = true;

          }
          if (newStatus === 103) {
            callback('onReady');
          }
         if (newStatus === 103) {
            callback('onError');
          }
        });
```
Merge pull request #175 from francoisTemasys/patch-1

Propagate transport fail state (e.g. 500)