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.

test/units/TEST-22-TMPFILES.03.sh

@@ -5,6 +5,9 @@
 set -eux
 set -o pipefail
 
+# shellcheck source=test/units/util.sh
+. "$(dirname "$0")"/util.sh
+
 rm -fr /tmp/{f,F,w}
 mkdir  /tmp/{f,F,w}
 touch /tmp/file-owned-by-root
@@ -190,6 +193,22 @@ F     /tmp/F/daemon/unsafe-symlink/exploit    0644 daemon daemon - -
 EOF
 test ! -e /tmp/F/daemon/unsafe-symlink/exploit
 
+#
+# 'C'
+#
+rm -rf /tmp/C-copy-failure-src /tmp/C-copy-failure-dst
+mkdir /tmp/C-copy-failure-src /tmp/C-copy-failure-dst
+printf payload >/tmp/C-copy-failure-src/file
+chmod 555 /tmp/C-copy-failure-dst
+
+(! runas nobody systemd-tmpfiles --create - <<EOF
+C     /tmp/C-copy-failure-dst    - - - - /tmp/C-copy-failure-src
+EOF
+)
+test ! -e /tmp/C-copy-failure-dst/file
+chmod 755 /tmp/C-copy-failure-dst
+rm -rf /tmp/C-copy-failure-src /tmp/C-copy-failure-dst
+
 #
 # 'w'
 #