parallelCollect method.

[daviddenton]

A solution to issue #334 - regarding async parallel error handling. Added new method, tests and documentation for it.

Generally, this is a version of parallel() which, instead of failing on the first error, continues and gathers all errors. The final callback is called with the error list/object and the result list/object.

I've now implemented this functionality across a few projects - it seems to be something that would be useful in the core.

[daviddenton]

Apologies - I've just noticed some annoying line break changes have got into the .md file and are rather confusing the diff view of the pull req. The interesting changes in the diff are at the bottom.

If it's a problem and you'd like me to resubmit the request, please let me know and I'll fix it and resubmit.

Thanks.

[iliakan]

Looks useful. The cuts of extra spaces in .md looks like is a reasonable change too =)

[caolan]

We need a more general solution to this problem. All async functions do early exit on error at the moment, so I don't want to introduce a special alternative for parallel alone.

[reggi]

Here's a general solution I just came up with: https://gist.github.com/reggi/11186741

var collect = function(callback){
  return function(){
    var args = Array.prototype.slice.call(arguments);
    args.unshift(null);
    return callback(null, args);
  }
}

Then within the call: var callback = collect(callback);

[aearly]

Reopening this for reference, because I think it's the best implementation of this feature. I'd like to figure out how to apply this more broadly.

[aearly]

Closing this in favor of #942. There are several things to figure out in order to support this type of behavior, then we can re-implement.