LPS-37928 Use the right portletId when hiding the default error message.

The portletId in the portletConfig object obtained through getPortletConfig() is the rootPortletId, but portlet_messages.jspf uses the portletId. Thus, for instantiable portlets the portletId should be obtained through portalUtil.getPortletId(request). This works in configuration views, because there the portletId of the configurationPortlet (86, non-instantiable) is used. In struts portlets, the portletConfig is properly obtained in the execute method of the super class.