[plt-scheme] Aggressive CGI error handler

I am writing a CGI script, and in testing, I discovered that 
MzScheme/the CGI library helpfully intercedes when a radio group is left 
empty.

Perhaps it is good practice to have a default value (and thus never have 
an empty radio group), but I'd like to handle this error myself. I don't 
want MzScheme dumping all the variables it knows about, and complaining 
"No binding for field `foo' in" ...

Is this something I can override? Do I need to wrap something like a 
"with-handlers" somewhere? It isn't clear to me from the documentation, 
but I might be looking at the wrong docs.

Thanks,
Matt