bug#8246: dired-create-directory should say at least one word in the case that it fails to do what it is supposed to

[Chong Yidong]

jidanni@jidanni.org writes:

> Please please please implement similar warnings to
>
>    # mkdir /
>    mkdir: cannot create directory `/': File exists
>    # mkdir /dev/null
>    mkdir: cannot create directory `/dev/null': File exists
>
> Even one murmur would be great!
>
> for
> + runs the command dired-create-directory

Fair enough.  Added to trunk.