a good custom error page will have search and navigation to the rest of the site, possibly relevant to the originally requested URL, but not much content unique to that page.
your category page, especially if it acknowledges the missing content that was requested, will look similar to a good custom error page.
the only difference is the error code supplied.
a 302 to a custom error page is a signal of an improperly configured server.
therefore your response to that request probably looks more like a "soft 404" response than a "temporary move" of your content, especially if more than one URL provides a similar response.