From UX POV, if a user tries to go to a project that doesn't exist either:

1. He received a link with a typo or something like that. You may try to fix the url for him;
2. You could show him a special page with project. Not the current "The page you were looking for doesn't exist."
3. You could encourage him to create a new project with "this" name  :D

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How do you deal with it on the other actions? If you need this in other places I would use a general rescue_from.