Supplying your own custom message via the “requiredMessage” property of the control is the best practice way of changing the message as it would ensure that both the client-side validation and server-side validation use your custom text.
I haven’t tested this, but you may try raising the ValidatorException with a different severity (the default is FacesMessage.SEVERITY_INFO) with something like this to affect how it is displayed:
throw new ValidatorException(new FacesMessage(FacesMessage.SEVERITY_ERROR, msg, null));
#webMethods#MWS-CAF-Task-Engine#webMethods-BPMS