[jdk21u-dev] RFR: 8313290: Misleading exception message from STS.Subtask::get when task forked after shutdown

[Daniel Hu]

Clean backport. Fixes misleading error message from StructuredTaskScope.get(). Affected test passes on linux x64

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Commit messages:
 - Backport ed0f75f2666f61ec3d8ccb78594a21db9f1be50a

Changes: https://git.openjdk.org/jdk21u-dev/pull/1720/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1720&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313290
  Stats: 27 lines in 2 files changed: 4 ins; 19 del; 4 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1720.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1720/head:pull/1720

PR: https://git.openjdk.org/jdk21u-dev/pull/1720