project view: highlight module group nodes in project view if there are files with errors under them (IDEA-171965)

Implementation of 'validate' from ProjectViewNode invokes 'update' method which won't call 'postprocess' method because 'isValidating' is true, so calling 'validate' on ModuleGroupNode clears ERROR_ATTRIBUTES key set in AbstractTreeNode.postprocess. So we need to override 'validate' method and don't call 'update' from it (IDEA-171966).

platform/lang-impl/src/com/intellij/ide/projectView/impl/nodes/ModuleGroupNode.java

@@ -91,6 +91,11 @@ public abstract class ModuleGroupNode extends ProjectViewNode<ModuleGroup> imple
     return someChildContainsFile(file, false);
   }
 
+  @Override
+  public boolean validate() {
+    return getValue() != null;
+  }
+
   @Override
   public void update(PresentationData presentation) {
     final String[] groupPath = getValue().getGroupPath();