sbox error message now lists the files that were created

which should make debugging faster.

Bug: 66921582

Test: ./out/soong/host/linux-x86/bin/sbox --output-root outs --sandbox-path out/.temp/sbox-work -c "cd __SBOX_OUT_DIR__ && mkdir asubdir && mkdir b && touch asubdir/child a b c d e f g h i j k l m n o p" outs/a outs/b outs/z # and observe the below output:

mismatch between declared and actual outputs
in sbox command(cd out/.temp/sbox-work/sbox343858828 && mkdir asubdir && mkdir b && touch asubdir/child a b c d e f g h i j k l m n o p)

  in sandbox out/.temp/sbox-work/sbox343858828,
  failed to create 2 files:
    b: not a file
    z: does not exist
  did create 16 files:
    a
    asubdir/child
    c
    d
    e
    f
    g
    h
    i
    j
    ...6 more

Change-Id: I75e37834c44d4279dec874701d67ce8bb01b872c