Attaching stdout, stderr to error objects

[deployable]

The stdout and stderr that are returned from an exec error are lost during error handling. The error itself doesn't always include complete context of the problem.

It would be useful if they could be attached to the error object that is passed back to the callback.

attach_streams_to = ( obj, stdout, stderr )->
  obj.stdout = stdout
  obj.stderr = stderr
  obj

return attach_streams_to( err, stdout, stderr ) if err

I guess the next level is using spawn rather than exec and controlling streams and the error completely.

[notatestuser]

There's an effort to use child_process.spawn in #38