HG: HG_Forward completion callback changes

[frostedcmos]

Currently HG_Forward()'s behavior is either to either invoke the completion callback (with error or success), or to return error from HG_Forward().

In CART/DAOS we end up having to manually invoke completion callback when HG_Forward() returns error in order to have a single place (completion callback) where all errors are being handled.

It would be nice as such to just have HG_Forward() always invoke completion callback for any error on its own, so that we don't have to recreate hg completion structures ourselves and have to manually trigger completions.