be consequent in requiring an existing directory

Before the fix sxcp -r sx://sth /tmp/dir/ would require /tmp/dir/ to be
created but sxcp -r sx://sth /tmp/dir would happily create the directory.

This commit makes the behaviour consistent, however the side effect is
that -r will not work nicely when copying single files.

The other solution is to go back to the old approach and create the
target dirs automatically.