GA-based js error logging. Fixes #660 for the time being.

index.html

@@ -182,7 +182,18 @@ _gaq.push(['_trackPageview']);
   var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
   ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
   var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
+
+  // javascript errors
+  var lastev = '';
+  window.onerror = function(message, file, lineNumber) {
+      var ev = ['_trackEvent', 'error', file + ':' + lineNumber, message + ''];
+      if (ev.join(',') !== lastev) {
+          _gaq.push(ev);
+          lastev = ev.join(',');
+      }
+  };
 })();
+
 </script>
 
 <!-- crazyegg -->