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. 
Target node not responding is the definition of a timeout error @vblagoje :)

AFAICT Sanne had a TimeoutException thrown by his own code inside a M/R task, so I think it would be better to wrap exceptions thrown by mappers/combiners/reducers in another exception.