make error messages occurring in generic WHEN hat scripts visible

history.txt

@@ -3264,6 +3264,14 @@ http://snap.berkeley.edu/run#cloud:Username=jens&ProjectName=rotation
 * GUI: Libraries Browser, thanks, Michael!
 * Blocks: Fixed out of bounds issue with “go back _ layers”, thanks, Brian Broll!
 
+170110
+------
+* German translation update
+
+170111
+------
+* Error handling improvements for custom drop-down submenus and generic WHEN hats
+
 
 == v4.10 === (in development)
 

threads.js

@@ -61,7 +61,7 @@ StageMorph, SpriteMorph, StagePrompterMorph, Note, modules, isString, copy,
 isNil, WatcherMorph, List, ListWatcherMorph, alert, console, TableMorph,
 TableFrameMorph, ColorSlotMorph, isSnapObject*/
 
-modules.threads = '2017-January-04';
+modules.threads = '2017-January-11';
 
 var ThreadManager;
 var Process;
@@ -361,6 +361,9 @@ ThreadManager.prototype.findProcess = function (block) {
 
 ThreadManager.prototype.doWhen = function (block, stopIt) {
     if (this.pauseCustomHatBlocks) {return; }
+    if ((!block) || this.findProcess(block)) {
+        return;
+    }
     var pred = block.inputs()[0], world;
     if (block.removeHighlight()) {
         world = block.world();
@@ -369,8 +372,6 @@ ThreadManager.prototype.doWhen = function (block, stopIt) {
         }
     }
     if (stopIt) {return; }
-    if ((!block) || this.findProcess(block)
-    ) {return; }
     try {
         if (invoke(
             pred,