update use of Futures to suit scala 2.13

[pjfanning]

Pull Request Checklist

• [x] Have you read through the contributor guidelines?
• [x] Have you signed the Typesafe CLA?
• [x] Have you squashed your commits?
• [ ] Have you added copyright headers to new files?
• [ ] Have you checked that both Scala and Java APIs are updated?
• [ ] Have you updated the documentation for both Scala and Java sections?
• [ ] Have you added tests for any changed functionality?

Fixes

Partial fix for https://github.com/playframework/play-iteratees/issues/16

Purpose

What does this PR do?

Removes the use of Future#onFailure due to scala 2.13 issues

Background Context

Why did you take this approach?

I've looked at the scala collections changes needed but there is still a lot of changes needed there. I thought I'd post the less awkward Future changes first.

References

Are there any relevant issues / PRs / mailing lists discussions?

https://github.com/playframework/play-iteratees/issues/16

I messed up https://github.com/playframework/play-iteratees/pull/19 trying to rebase it

[alexflav23]

@cchantep Any chance we'll see a 2.13 version soon? Not sure if there's anything I can do to help it along.