Catch url for script errors when we can

I'd really like to better understand what all these script errors we're getting are.  My investigation last week showed that in FireFox at least, we can get the URL for script errors. I gave this feedback to New Relic, but have no reason to think this will necessarily be added to their tools in the near future.

This attempts to wrap the onerror logic myself.  It's a little bit scary in that this will behave differently on localhost (which doesn't have new relic), and because we're trying to catch a case that I don't have a repro for.