bug#24958: Please make "omitting directory" message clearer

[Samuel Thibault]

Hello,

Every year, my students are completely taken aback when trying to copy a
directory:

$ cp /foo/bar .
omitting directory /foo/bar

The problem is that the error message is not explicit at all about what
issue cp actually encountered: "yes it is a directory, so what?!"

Could this be changed into for instance:

$ cp /foo/bar .
not recursing into directory /foo/bar by default

which will help them figure out they just need to use -r.

Samuel