Add comments, diganostics, pass back some errors from funcs.

Add comments, diganostics, pass back some errors from funcs.
Merge of master-melb:xfs-cmds:24197a by kenmcd.

  When tracking down an error msg from fsstress, I had no idea what
  was going on. So I have added a bunch of comments to the code
  and added some more diagnostics (in verbose mode). Have also passed
  back some failures which weren't directly before.
  Hopefully this will not change the ops which are called for
  a given rand-seed run.