New error events

Hi folks,

I'm starting to come to the conclusion that we need some ways
to implement error notification as distinct from either setLocal/setRemote
or state notification. Here are some examples of events that don't
seem to me to have a logical way to report to the programmer:

- ICE allocation failed (e.g., couldn't actually allocate sockets for
each component.) Obviously, you could report that allocation
is *done* but it's not like a naive application is going to notice
that candidates are missing.

- DTLS-SRTP handshake failed.

- The ever-famous "internal error".

I would suggest that we have some sort of "onerror" event, perhaps
with a numeric error enum and a string that is explanatory.

Thoughts,
-Ekr