[webidl] Define DOMException.prototype.toString() (as this.name + ": " + this.message) (#93)

I can't find `toString` in http://heycam.github.io/webidl/#es-DOMException-prototype-object but it's supported by Blink, Gecko and WebKit as `this.name + ": " + this.message`. IE11 appears to stringify as either the name or the message, I can't tell because they're the case I tested.

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https://github.com/heycam/webidl/issues/93