Invalid paths cause the response to hang

[AngusMorton]

We're seeing people hit our Marko Run service with the path // which causes the request to hang until it times out.

Specifically, I think it's throwing here because new URL("//", http://www.example.com") throws.

Potentially, a fix is to use URL.parse and handle a missing URL in the fetch handler by falling back to a 500 page.