Zend_Form: Subform Validation

Hi all,

Nice easy question (I hope) but I havent been able to figure it out
from the code or the documentation.

How would you go about displaying a custom error message when a sub
form fails validation? i.e. I've overloaded the subform's isValid()
method to add some custom validation thats reliant on a combination of
all the fields in the subform, if this fails I'd like to pass an error
message to surround the subform as with individual form elements.
Output and decoration issues aside, I can't even see how to log the
error anywhere I'm afraid.

Thoughts?

Cheers,

Steve