machine: propagate foreign image clone directory errors

unprivileged_clone() returns success when mountfsd_make_directory() fails.
Consequently, callers can report a successful clone even though the
destination directory was never created.

Return the original error instead of zero so the failed clone is
reported to the caller.

Add a regression test that places a regular file without an image suffix
at the destination path. Such a file is ignored by image discovery but
prevents directory creation. Check that cloning reports failure without
changing the file, then succeeds after the conflicting file is removed.

src/shared/discover-image.c

@@ -1725,7 +1725,7 @@ static int unprivileged_clone(Image *i, const char *new_path) {
                         /* flags= */ 0,
                         &new_fd);
         if (r < 0)
-                return 0;
+                return r;
 
         /* Mount new image */
         _cleanup_close_ int target_fd = -EBADF;