Logged In: NO

Hmm ... I've found a neat workaround in case there's no
other solution ... a script which identifies the missing fields
and adds a search string to the missing_fields_redirect URL.

The error page can decode the search string to report on
which fields are missing:

In the form ...

function validate() {
errorURL = "baseURL/form_error.html?"
if (document.forms[0].email.value == "" ) errorURL =
errorURL + "Email%20Address%20%20";
if (document.forms[0].Contact_Name.value == "" )
errorURL = errorURL + "Contact%20Person%20%20";
if (document.forms[0].Phone.value == "" ) errorURL
= errorURL + "Phone%20Number%20%20";
document.forms[0].missing_fields_redirect.value =
errorURL
}

... and in the error page ...

document.write("The following fields on your form must be
completed<br><br>"
+ unescape(location.search.substr(1))
)