RFR: 8312436: CompletableFuture never completes when 'Throwable.toString()' method throws Exception

[Viktor Klang]

Primarily offering this PR for discussion, as Throwables throwing exceptions on toString(), getLocalizedMessage(), or getMessage() seems like a rather unreasonable thing to do.

Nevertheless, there is some things we can do, as witnessed in this PR.

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Commit messages:
 - CompletableFuture should be more resilient against misbehaving exceptions

Changes: https://git.openjdk.org/jdk/pull/18988/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18988&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312436
  Stats: 63 lines in 2 files changed: 52 ins; 2 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/18988.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18988/head:pull/18988

PR: https://git.openjdk.org/jdk/pull/18988