Sanity check that only active timers are stopped

src/refit/scheduler/REFITScheduler.java

@@ -119,7 +119,10 @@ public class REFITScheduler extends Thread implements REFITSchedulerFacade {
 		if (this != Thread.currentThread()) {
 			throw new IllegalArgumentException("Timeout must only be set by the task itself!");
 		}
-		timeouts.remove(timer);
+		boolean removed = timeouts.remove(timer);
+		if (!removed) {
+			throw new IllegalStateException("The timer is not active!");
+		}
 	}