Improve error handling in case the HTML page or QUnit can't be loaded

[matz3]

Currently there's an exception in case the HTML page or QUnit can't be loaded

TypeError: Cannot read property 'begin' of undefined

This should be improved within the plugin to log a proper error message.
In addition the test page should be reported as failing and the runner should continue with the next page. Currently it just stops and runs into a timeout.

The detection should differentiate the two scenarios

• HTML page can't be loaded at all (404)
• QUnit is not loaded (typeof QUnit === "undefined")