When I use the revert button on a page with only one contributer I get the error measage:
You do not have permission to do that, for the following reason:
Cannot revert edit; last contributor is only author of this page.
Maybe it's more generic to say:
It's not possible to do that, because of the following reason:
It's easy to change and not important.
For delete an not existing page is the measage
Could not delete the page or file specified. (It may have already been deleted by someone else.)
mayb' something like this is also a solution.