format_status analog for error_info

[Andrew Thompson]

So, with the release of R13B4 I was excited about the new format_status
callback for gen_server and gen_fsm. Unfortunately I guess I
misunderstood the intent of the patch (and foolishly didn't look at the
implementation) and so the functionality is not what I'd hoped.

What I was hoping for was to be able to use that callback to also modify
the State information fed into error_info so that the problem of a
process with massive internal state wasting vast amounts of memory
sending its internal state to the SASL error_logger could be avoided if
you used that callback to trim out the large and useless data (I don't
care to see the giant binary my code was working on 90% of the time).

Any thoughts on the best way to do this. Right now I see 3 options:

* Piggyback on the terminate() callback. If the return value is of a
  special form (maybe just {ok, State}) then don't ignore the result and
	use the new State as the argument passed to error_info()
* Add another new callback called something like format_error that gets
  called (if exported) before error_info and can modify the state data.
* Use format_status (maybe passing 'terminate' as the Opt, instead of
* 'normal')  and look for the [{data, [{"State", State}]}] part
  of the output (or something else?).

I know that lots of people struggle with error_logger getting swamped by
large crash messages when procesesses with large state die (facebook
even put it in one of their slide decks), so I do intend to fix this -
I'm just not sure of the better way (and the way that maximises the
chances of a patch being accepted).

Thanks and congratulations on the new release,

Andrew