PAXWICKET-449 replaced NPE with meaningfull message
(cherry picked from commit 185b86e4aac9d83b61315635723c77318dfc220e)

Signed-off-by: Andreas Pieber <anpieber@gmail.com>

Conflicts:
	service/src/main/java/org/ops4j/pax/wicket/internal/extender/BundleDelegatingExtensionTracker.java