Track 404 errors as events with Google Universal Analytics, capturing the referrer as well.
<script type="text/javascript">
// Track 404 errors with Universal Analytics
if($('body').hasClass('error404')) {
ga('send', 'event', 'error', '404', 'page: ' + document.location.pathname + document.location.search + ' ref: ' + document.referrer, {'nonInteraction': 1});
}
</script>