improve custom-protocol-handler.js

As documented in [the Undetectables](https://github.com/Modernizr/Modernizr/wiki/Undetectables)
navigator.registerProtocolHandler was stubbed in webkit for a while, and doesn't actually do anything there.
This test forces a failure and ensures it's the right type of failure.

Android 2.* showed this behavior previously, and this test fixes that false positive.