Add a rollback() DSL to force a rollback for transacted exchanges

When you want to rollback a TX with Spring TX you need to throw a runtime exception.

It would be nicer with a rollback DSL that can set a marker on the exchange and then the Spring TX interceptor can notice this and force a rollback.

As the rollkback() should also work under non TX conditiions it should throw a runtime exception for non TX exchanges.
Or we should just add a new RollbackExchangeException that will be thrown.