MediaError missing message property

[nikmartin]

https://github.com/Microsoft/TypeScript/blob/bb23e9601f5ea0b4fe1e97545ef01535fa18a23d/lib/lib.d.ts#L11664

According to spec The MediaError object should have a message:string property:

https://html.spec.whatwg.org/multipage/media.html#dom-mediaerror-message

[mhegazy]

PRs welcomed. You can find more information about contributing lib.d.ts fixes at https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes.