That message isn't added until after the action is invoked. Right now there is no way to get rid of it, but you can customize it with the message key "org.jboss.seam.TransactionFailed".
Actually, looking at the code, you could subclass the Seam phase listener and override addTransactionFailedMessage() if it is absolutely critical.
Thank you for the feedback Norman, even though I do not like much this approach since it does not give the flexibility to narrow down the error message. I guess the right approach would be to handle the process manually.