coll: remove errflag propagation

[hzhou]

Pull Request Description

Propagating errors in collectives rather than locally aborting adds tons of maintenance cost with very little proved benefits. I personally often waste days in debugging simply due to the difficulty of pin-pointing the source of error. With local aborting, our error ring shows the error traceback. The error propagation loses this error trail, and propagates the error with limited information to remote places -- that is just evil.

Of course, this is a draft PR for the purpose of discussion.
[skip warnings]

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