Joins could not be atomic

[GregMacdonald]

We use joins where we are collecting a list of objects being fetched. If any fetch fails, the aggregate operation is failed and our list ends up empty, but it would be better to get the list without objects that fail.

It would be good if a join could return the good and the bad. So, maybe something like:

typedef id(^joinPromiseCallback)(NSArray *values, NSArray *errors));

• (KSPromise *)eventually:(joinPromiseCallback)callback