bug#30285: dired-do-chmod vs. top line of dired

[Drew Adams <drew.adams@oracle.com>]

> > Not sure how helpful or necessary that is.  It's liable to
> > not be helpful (that use case needs no special advertising).
> > And it might even confuse things.  I think it just gets in
> > the way of the message, which is, "You are not on a file line."
> 
> This message is absolutely wrong, it doesn't describe the state that
> causes the error message.  It has a bigger scope than just file lines,
> it works with marked files, etc.

You're right about that.

> So more correct message would be like this:
> 
>   “No files selected.”

That's better, but it risks confusion over the notion of
"selection", especially in the context of marking or using
marks (a "selection" of files).

This might be better: "No files specified"
Or this: "No files chosen"

> Oh, and I discovered that the current state is much worse than I
> thought:
> 
> 1. load dired-x
> 2. type ‘M-<’ to go to the first line
> 3. type ‘!’ (dired-do-shell-command)
> 
>   “Wrong type argument: stringp, nil”

Meme combat - same thing.

The return value of `dired-get-marked-files' needs to be tested as
soon as it's available.  When it is nil we must not call
`dired-read-shell-command' or do anything else - just raise a
`user-error'.

This is a general change that needs to be looked for and made
wherever appropriate.