FIX: Creating a resizable panel using an HTML code fragment starting …

…with a comment will break

As mentioned in adobe#6252, the original error msg isn't really helpful. 
The modified msg could be improved as well, though:
Since the content of the panel may be a **HTML fragment** (usually starting with a DIV node), mentioning that ```the **first** (root) DOM node must have an ID that will be used as a pref key``` would help.

Nevertheless, the mangled code in WorkspaceManager.js isn't reliable since it will _break_ when the panel content **starts with a html comment**. Such a comment may be necessary when  a plugin like HTMLhint is used. On a "panel" code fragment, this plugin attests an error due to a missing doctype. To disable this error, one may put a comment at the top of the html code: ```<!--htmlhint doctype-first: false-->```. WorkspaceManager.js will fail creating the panel, then. The proposed modification fixes such an issue.

**sample test code for a resizable panel**
```
<!--htmlhint doctype-first: false-->
<!--This comment is of no use but for testing purpose-->
<div id="my-very-special-testing-panel"><span style="color: red">Elvis</span> was here!</div>
```

src/view/WorkspaceManager.js

@@ -166,8 +166,8 @@ define(function (require, exports, module) {
      */
     function Panel($panel, minSize) {
         this.$panel = $panel;
-
-        Resizer.makeResizable($panel[0], Resizer.DIRECTION_VERTICAL, Resizer.POSITION_TOP, minSize, false, undefined, true);
+        var $rootNodeWithID = $panel.filter("*")[0];
+        Resizer.makeResizable($rootNodeWithID, Resizer.DIRECTION_VERTICAL, Resizer.POSITION_TOP, minSize, false, undefined, true);
         listenToResize($panel);
     }