This adds a warning in the case that the default action is not null but
also not callable which can happen when the supplied method is not
public.

Caching had been completely broken (disabled) for caches with events
because the default event handler (cache::_useCache()) was protected and
thus couldn't be executed by the event handler. This was broken in
c59b3e00

This reverts commit 524df576.

Conflicts:
	inc/lang/bn/lang.php
	inc/lang/fr/lang.php