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. 
Aha, I get you! Yeah, it makes sense to peel off ExecutionException I think. No reason for it to be there if after all we throw CacheException to the client. Let me experiment and see what the effect is!