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

[Juri Linkov]

>> Since `dired-mark' from the top line followed by the command in question
>> is not obvious for users, we could provide a hint in the error message,
>> i.e. mention the availability of ‘m’ on the top line with such message:
>> “You can type `m' here to mark all files for this operation”.
>
> 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.  So more correct message would be like this:

  “No files selected.”

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”