Thanks for responding...
After a bit more testing I have found that my script is getting control, but with an empty parameter set as expected. I am able to retrieve the cgi_error "413 Request entity too large" from cgi_error but am finding that I cannot post an html msg to the user. I seem to be falling right through the code that puts out that error msg, produce the "page cannot be displayed" error, but am able to subsequently send myself an email with the 413 error. I believe the code that puts out the html error msg is good - am using it for other errors.