cp: adapt error message if destination directory doesn't exist

[cakebaker]

When trying to copy a file to a non-existing directory, uutils cp currently shows the following error:

$ cargo run cp a.txt no-such/
cp: 'a.txt' -> 'no-such/': Is a directory (os error 21)

GNU cp, on the other hand, shows the following error:

$ cp a.txt no-such/
cp: cannot create regular file 'no-such/': Not a directory

This PR adapts the error message to match the one from GNU cp.

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/tail/symlink is no longer failing!

[tertsdiepraam]

Do you know why this check for the separator does not show up anywhere else in the code? Surely there's some place where we have to check for that so that we can interpret this correctly as copying to a directory or is that not implemented?

I think we might be missing something somewhere else that is causing the different message...

[sylvestre]

@cakebaker ping?

[cakebaker]

I don't know why there isn't such a check for the separator.

[cakebaker]

Closing this PR, it has been fixed in a different way in #5780