Did you throw a faces exception in the getAsObject method with the message you need displayed or another type of Exception?
I'm throwing a ConverterException and the seam is not catching it.
I'm using SEAM 2.0CR1
If your converter isn't marked @BypassInterceptors, then exceptions get wrapped, I think. So maybe you just need to add that annotation.
That is it, ByPassInterceptors did the trick :-)