Consider using AggregateError for tee() cancelation reasons

[domenic]

Currently we put the cancelation reasons into an array. But since tee() was invented, AggregateError has shipped, which is made for this purpose.

I'm not sure this is worth the breaking change (especially since it'd be pretty hard to measure usage of the array form), but I thought it'd be worth considering...

[MattiasBuelens]

I'm leaning towards "yes". So far, the only time where I was interested in the array of errors from a tee'd stream was when I wanted to convert it into an AggregateError myself. I'd be happy if I can remove that hack out of my own code. 😛

But indeed, it might be difficult to know if/how this is being used in the wild. 😕