[BEAM-659] Cherry-pick #2985, WindowFn error message improvement, to release-2.0.0

[kennknowles]

Be sure to do all of the following to help us incorporate your contribution
quickly and easily:

• Make sure the PR title is formatted like:
  [BEAM-<Jira issue #>] Description of pull request
• Make sure tests pass via mvn clean verify.
• Replace <Jira issue #> in the title with the actual Jira issue
  number, if there is one.
• If this contribution is large, please file an Apache
  Individual Contributor License Agreement.

---

This cherry-picks #2985 which adjusts the API for WindowFn to allow providing a meaningful error message to users.

[kennknowles]

run flink validatesrunner

[kennknowles]

(just checking that there is no neglected ValidatesRunner test that this affects - the precommit is known to be good)

[kennknowles]

Even though precommit ran out of time in the Dataflow examples, the examples because with only 2 minutes left on the clock. I don't think this was caused by that, but by overall build time growth. I have also confirmed that there is not even a compile time incompatibility here, so a runtime incompatibility is extremely unlikely.

I'll note that the Flink ValidatesRunner passed for simple coverage, then I'll rebase and kick precommit and
Dataflow ValidatesRunner for added signal there.

[kennknowles]

run dataflow validatesrunner

[kennknowles]

I think that unfortunately the impact of this is not sufficient to make a backwards incompatible change right now. We'll do it in a compatible way.