[webrtc-pc] legacy error handling

fippo has just created a new issue for https://github.com/w3c/webrtc-pc:

== legacy error handling ==
mostly editorial issue

https://w3c.github.io/webrtc-pc/archives/20170515/webrtc.html#error-handling
talks about legacy extension error handling. This is somewhat separate from https://w3c.github.io/webrtc-pc/archives/20170515/webrtc.html#legacy-interface-extensions and should probably be moved there.

It also says
> Legacy-methods may only throw exceptions
but the legacy description of the legacy method never mentions they throw.

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1236 using your GitHub account