ACT-2112 JtaTransactionInterceptor hides exception

It is possible that JtaTransactionInterceptor swallows original
exceptions, when an exception is thrown on rollback.
ACT-2112 JtaTransactionInterceptor hides exception

When exception occurs on rollback, the original causing exception is lost.
FIX: log.error the original exception upon throwing the new one.
Manual merge for pull request 389
Merge branch 'master' of https://github.com/Activiti/Activiti