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

[jidanni]

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

please.