addIceCandidate needs an error callback

The spec says about addIceCandidate()

An exception with an RTCError object of type INVALID_CANDIDATE_TYPE is
thrown if candidate parameter is malformed.

I believe this doesn't match with our agreed error handling principles:

Say I add a candidate with a bogus candidate string (e.g., a missing "typ"
field or something).

Like malformed SDP, this should be handled with an error callback.

-Ekr