Component.getDefaultModelObject() wraps in RuntimeException instead of WicketRuntimeException

If a model throws an exception when wicket tries to get the model value during rendering using Component.getDefaultModelObject() the original exception will be wrapped in a vanilla RuntimeException. This will then be wrapped again in a WicketRuntimeException. Perhaps there is a reason for this behaviour that I'm not aware of, or perhaps it's just accidental? I would expect the getDefaultModelObject() to wrap directly in a WicketRuntimeException, which will propagate the wicket stack properly without further wrapping.

My use case is a SessionTimeoutException (originating from a timed out session to a third system) that depending on which path wicket has travelled when the exception is thrown, it is sometimes wrapped directly in a WicketRuntimeException and sometimes wrapped in a RuntimeException wrapped in a WicketRuntimeException. This can be handled by the top level exception handler of course, but requires more complexity and may offer a few runtime WTFs...

Changing Component.getDefaultModelObject() to throw a WicketRuntimeException should be fairly safe since it is a RuntimeException and it will avoid the redundant wrapping.

Unit test quickstart attached.

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