console: throw when no such label exists in console.timeEnd

Test included.

lib/console.js

@@ -49,7 +49,11 @@ exports.time = function(label) {
 
 
 exports.timeEnd = function(label) {
-  var duration = Date.now() - times[label];
+  var time = times[label];
+  if (!time) {
+    throw new Error('No such label: ' + label);
+  }
+  var duration = Date.now() - time;
   exports.log('%s: %dms', label, duration);
 };