checkRendering message should also mention XHTML tags as a possible cause

When working in development mode with warnings turned on, it's not uncommon to get an error message like the one below when components are added in code, but not found in markup.

"org.apache.wicket.WicketRuntimeException: The component(s) below failed to render. A common problem is that you have added a component in code but forgot to reference it in the markup (thus the component will never be rendered)."

This is useful, but doesn't go far enough when assisting new developers. Another common cause that I've seen often is a component is defined and added in a parent class (e.g. AbstractBasePanel), but the child class markup file is missing the <wicket:extend> elements.

I would like to see the messaging include a statement like: "If your components were added in a parent Page or Panel, make sure the markup for your child MarkupContainer includes the appropriate Wicket XHTML tags such as <wicket:extend>."