addIceCandidate needs an error callback

In our work on implementing the current spec, we've run into a problem 
with the fact that addIceCandidate doesn't have any callback parameters. 
It is quite possible, for example, to enqueue an addIceCandidate that 
looks perfectly good at the time it is enqueued, but discover that it is 
invalid when it comes time to execute it. This is quite plausibly 
something applications would want to know about, if for no other reason 
than to simplify troubleshooting.

To handle this situation, I would propose updating the signature to be 
congruent with the other queued operations; to wit:

void addIceCandidate (RTCIceCandidate candidate, VoidFunction 
successCallback, RTCPeerConnectionErrorCallback failureCallback);

If we make the callbacks optional, then this change is completely 
backwards compatible with anything using the current specification.

Sound good?

/a