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Instead, I opted to break into library/Error.php, and replaced the default message with basic HTML which is the same as this sample rendering of the new error page:
People see an identity of the website, why they are unable to access it, and the steps they should take to try and resolve the problem (wait 5 minutes or email an admin.)
Wouldn't you say this is far more helpful? Now, this wasn't hard for me to implement, but it would have been great if we could enable this kind of modification with a simple tweak in a dedicated Template file or by defining a config variable in the config.php file.
I know this is only small and doesn't really offer any more day-to-day user features to XenForo, but I can't imagine this would be especially difficult to implement from a programming perspective.