Add XHR error handling for offline

[bradgreens]

My use case is if the browser becomes offline and the XHR requests begin to error, my JS app continues to run and watch for online availability.

This adds a hook to push null into the latency array if the XHR request responded with the error event, allowing the net.latency.on( 'end' ) callback to receive null information and conclude there is no connection. Not sure if these needs to accommodate for intentional server errors, but it's an improvement in my case.