bug#36831: enhance 'directory not empty' message

[Alex Mantel]

i couldn't use move:

    $ mv thing/ ../things
    mv: cannot move 'thing' to '../things/things': Directory not empty

An i can not move it. i do not understand why. I have to google and find
at stackoverflow:

    Though its man page doesn't document it, mv will refuse to rename a
    directory to another directory if the target directory contains files.
    This is a good thing in your case because you turn out to want to
    merge the content of the source into the target, which mv will not do.

https://askubuntu.com/questions/269775/mv-directory-not-empty

Ah, the target directory does exist! Hmm... But i'd like the message to 
be like:

   $ mv thing/ ../things
   mv: cannot move 'thing' to '../things/things': Targetdirectory not empty

                                                  ^ this little thing here,
                                                    it explains everyting.

Change text from 'Directory not empty' to 'Targetdirectory not empty'.