[webidl] Add a PermissionDeniedError (#99)

[Push-API defines](https://w3c.github.io/push-api/#dfn-permissiondeniederror) a `PermissionDeniedError`  as: _The operation failed because the user denied permission to use the API._

I'd need to use something similar for the Generic Sensor API. Think you could add it to the Spec?

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