bug#51883: 29.0.50; Command to get accidentally deleted frames back

[Gregory Heytings]

> > An error, something like "You didn't specify a frame number."
>
> What's different in this case, compared to commands that do use 
> (interactive "p")?  Why is raising an error here better than doing what 
> at least some people expect?

They will not expect it if the docstring (and the error message) mentions 
that an explicit numerical argument must be given.  And I don't think that 
"undelete the fourth most recently deleted frame with just C-u" is 
something anyone would expect, in fact I think it would be more confusing 
than helpful.

Moreover I want to keep the C-u prefix free for later use.