fix: Forward `poll_ready()` errors to the actor and possibly finish

[sdroege]

PR Type

Bug Fix

PR Checklist

Check your PR fulfills the following:

• [ ] Tests for the changes have been added / updated.
• [x] Documentation comments have been added / updated.
• [ ] A changelog entry has been made for the appropriate packages.
• [x] Format code with the latest stable rustfmt

Overview

It's not generally supported to poll a Sink again after it returned an error anywhere, and e.g. SinkMapErr from the futures crate is simply panicking if this happens.

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cc @gdesmott

[robjtede]

Could you contrive a test that would fail on master ?

[sdroege]

Yes, if this approach makes sense and you don't want to solve the underlying problem (which AFAICS requires breaking API changes)

[robjtede]

To make progress, this PR needs a test and a changelog entry.