How can we give Admins more control on what is shown for non-existent pages such as contribution pages
This same issue bothers me because I see so many fatal errors in logfiles across multiple servers when all that is happening is that a contribution page has been disabled / removed. I'd love to see it addressed in core if possible; CiviCRM should be capable of handling invalid inputs without showing the end user an unthemed, unhelpful error message.

One extension does already address this - ca.bidon.reporterror. It includes an option to detect those fatal errors (a user hitting a contribution page with invalid ID, or a disabled contribution page, or with an expired session) and redirect them to a default contribution page. This is a big improvement on the default behaviour! However, it's not included in core, and it comes bundled with other behaviours which might not be what all sites want.

Ideally, I'd like to make progress on tickets like CRM-19157: Replace CRM_Core_Error::fatal() with CRM_Utils_System::permissionDenied() so that we can drop the ugly (to users) and noisy (to sysadmins) "fatal" approach to handling invalid inputs in favour of a more "controlled landing" when things go wrong - which can both deliver a helpful and on-brand experience to the user seeing it, and reduce the level of log noise which results from the use of CRM_Core_Error::fatal() as a technique to bail out.

Instead we should deliver be delivering a nuanced experience to both users and admins: where we currently have CRM_Core_Error::fatal() we should throw an exception with information about what went wrong, and then CiviCRM can handle those exceptions appropriately. Broken contribute URL? Direct viewer to a default contribute page! User triggered IDS / DDOS / flood control? Maybe we can dispense with the pretty theme ...