I am new here so I don't have enough reputation to comment on answers and I apologize if I am doing this wrong, but the accepted solution will unfortunately not be able to accomplish what you are looking for.
I was able to demonstrate what I mean by having the script from the JSFiddle above run once the DOM is ready (there will be no alert but still an error in the console). Here's a little more detail on what happens currently with that fiddle:
// running with the No wrap - in <head> option
var frame = document.querySelector('iframe'); // null, the iframe isn't there yet
try {
frame.src="http://wedesignthemes.com/themes/redirect.php?theme=wedding";
} catch(e) {
// TypeError here, no hints about the iframe :(
alert('Error!');
}
The exception that is being caught has nothing to do with the iframe, it is actually a type error from trying to set the src property on a null value.
What you really want to do is catch the error inside of the iframe (when the sandboxed script tries to access window.top), but this is not possible because of the Same-origin policy. Btw, setting the "allow-same-origin" sandbox flag only has any effect when the iframe content is being served from the same origin as the top level document. E.g. as soon as the src or location of the iframe is changed to a different origin, there's no way to touch anything inside.
There are ways to communicate across iframe boundaries, such as with window.postMessage or the older and hackier way of using the iframe's location.hash, but I am assuming you can't affect the source of the page going into your iframe. (A nice developer would of course be open to suggestions and see that a feature like this could be useful.)
The only way that I was able to catch this error without violating any browser security policies was to set the allow-top-navigation sandbox flag and then use the window.onbeforeunload handler in the top level document to catch the navigation attempt from the child iframe. I would never recommend this because the user experience is awful. There is no way to prevent the navigation without prompting the user about whether they want to leave the page or not. Proof of concept below:
<iframe id="myframe" sandbox="allow-scripts allow-top-navigation"></iframe>
<script>
var url = "http://wedesignthemes.com/themes/redirect.php?theme=wedding",
frame = document.getElementById("myframe"),
listener;
listener = window.addEventListener("beforeunload", function(e) {
e.preventDefault();
e.stopImmediatePropagation();
// The iframe tried to bust out!
window.removeEventListener("beforeunload", listener);
return "This is unavoidable, you cannot shortcut a " +
"navigation attempt without prompting the user";
});
frame.src = url;
</script>
So unfortunately I can't find any ways to do this nicely in current browser implementations without help from your 3rd party content developer. I read some interesting things in the HTML5 spec that might allow us to do things like this in the future (and am unfortunately maxed out on my number of links I can insert here), so I would keep an eye out as things progress.
