tmpfiles: propagate copy errors after opening target

Opening the destination only proves that it exists. If copy_tree_at()
failed for any reason other than an existing destination, report that
copy error.

Reproducer:
  name=tmpfiles-copy-bug.$$
  src=/tmp/$name.src; dst=/tmp/$name.dst; conf=/tmp/$name.conf
  mkdir "$src" "$dst"
  printf payload >"$src/file"
  chmod 500 "$dst"
  printf 'C %s - - - - %s\n' "$dst" "$src" >"$conf"
  systemd-tmpfiles --create "$conf"

Before:
  exit=0
  copied=no

Follow-up for 8f6fb95cd0.

src/tmpfiles/tmpfiles.c

@@ -2238,6 +2238,9 @@ static int copy_files(Context *c, Item *i) {
                 return log_error_errno(errno, "Failed to openat(%s): %m", i->path);
         }
 
+        if (r < 0 && !IN_SET(r, -EEXIST, -EROFS))
+                return log_error_errno(r, "Failed to copy files to %s: %m", i->path);
+
         if (fstat(fd, &st) < 0)
                 return log_error_errno(errno, "Failed to fstat(%s): %m", i->path);