Can you please provide more details on the problem?
PaginatingDataModel extends SerializableDataModel
Usually in walk method we execute SELECT statement and if in this statement have some problem (syntax, DB Lock ...) we catch this and send message to user with addMessage, but message never shown on page.
Just put in walk method
new FacesMessage(severity, message, details));
This won't work well as components are rendered sequentially, so h:messages/rich:messages will render what has been queued by that moment and then your code adds messages.
Either place messages component after the table or use another style of errors reporting (e.g. throw exception).