better handling of non visible ideas in portal

When a user goes to a URL in the Ideas portal (e.g. https://big.ideas.aha.io/ideas/A-I-8311), if that Idea is not visible, it would be helpful if I could customize the error message.

Generically, the error should say that the Idea is not currently available in the portal, rather than showing a 404 error.

Specifically, I would like to be able to customize that message to say something like, "This Idea is currently under review.  Contact our Product Management team at pm-team@example.com to discuss."