Unable to test for system exceptions in requests

[JoshTumath]

While creating our Alexa skill, our team noticed that, if we forget to emit :responseReady when starting to play something in the audio player, the Alexa device will call a the Unhandled handler with the following request (see docs):

{ type: "System.ExceptionEncountered",
  requestId: "amzn1.echo-api.request.170b9121-04e5-4794-826d-1c00f51dcc8c",
  timestamp: "2017-10-02T11:01:02Z",
  locale: "en-GB",
  error:
   { type: "INVALID_RESPONSE",
     message: "An exception occurred while dispatching the request to the skill." },
  cause: { requestId: "amzn1.echo-api.request.cf8066e8-c7d8-4324-be51-f3c0d1ff9709" } }

So we changed our Unhandled handler to look a bit like this:

  Unhandled() {
    if (this.event.request.type === 'System.ExceptionEncountered') {
      // Log the system exception...
    }

    this.emit(':tell', 'Sorry, there was an error.');
  }

We would like to test how we handle system exceptions in our skill. However, as far as I know, there is no way within the testing framework to simulate an error in the request.