ObjectList: Fixes

* Fixed update of a warning icon for multi-part object, when some part with errors was deleted
 * Fixed a crash when selecting non-Object and non-Volume item (in get_mesh_errors_info() was missed check of the obj_idx)

+ Follow-up 1cc7b4ba97 - some code refactoring:
  * fixed RepairedMeshErrors.merge()
  * get_mesh_errors_count() renamed to get_repaired_errors_count()

src/slic3r/GUI/ObjectDataViewModel.cpp

@@ -1786,6 +1786,14 @@ bool ObjectDataViewModel::HasWarningIcon(const wxDataViewItem& item) const
     return node->has_warning_icon();
 }
 
+void ObjectDataViewModel::UpdateWarningIcon(const wxDataViewItem& item, const std::string& warning_icon_name)
+{
+    if (warning_icon_name.empty())
+        DeleteWarningIcon(item, true);
+    else
+        AddWarningIcon(item, warning_icon_name);
+}
+
 } // namespace GUI
 } // namespace Slic3r