You can add an onerror attribute to your iframe, e.g.:
	<iframe src="..." onerror="..."></iframe>
	Here's a live example that uses two different methods using jQuery (which is included with in5 output):
	https://jsfiddle.net/0z9mu8Lb/
	I did some more testing and found the onerror event is only firing properly in Firefox.
	These solutions offer a cross-browser method: https://jsfiddle.net/justinputney/jpaaytc4/2/
	Essentially, it adds a navigator.onLine check inside the onload attribute of the iframe. If offline, the onerror attribute code is evaluated.