grib save statistic

Alterations to saving logic for time processed statistics

Now:
  if time processed and stat known
    set value
  elif time processed and stat not known
    set value to 'missing
  elif not really a time processed cell method (else)
    raise exception
This try/catch is throwing away the explanatory messages which you create at lines 441/444 + replacing them with a single fixed message.  Wouldn't it be better to include the message from the inner error within this one ?