[webrtc-pc] Order of RTCError parameters (#2111)

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

== Order of RTCError parameters ==
Please forgive the drive-by comment; we're discussing [a related issue](https://github.com/tc39/proposal-promise-any/issues/14) over at TC39 and I noticed this.

The [RTCError constructor](https://w3c.github.io/webrtc-pc/#rtcerror-interface) takes two arguments, `message` and `init`, where `init` is used to provide values with which to initialize the new object.

The [OverconstrainedError constructor](https://w3c.github.io/mediacapture-main/#overconstrainederror-constructor) from the media capture API similarly takes two arguments, `constraint` and `message`, where `constraint` is used to provide a value with which to initialize the new object.

These are inconsistent in their order. Might the order of the parameters to RTCError be switched? This would also allow making the `message` parameter optional, as it is for most other error constructors (including `DOMException`).

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/2111 using your GitHub account