ISPN-4677 Map/Reduce job fails with the wrong explanation on TimeoutException

Master only. See https://issues.jboss.org/browse/ISPN-4677 for discussion and rationale. 
Throwing the outer ExecutionException doesn't provide any additional information though.  It just exposes underlying implementation details showing that we most likely use an ExecutorService is all.  Think what I was trying to say might have got lost in translation.