RFR: 8313290: Misleading exception message from STS.Subtask::get when task forked after shutdown [v2]

[Alan Bateman]

> This is a small issue with StructuredTaskScope. If the owner attempts to fork a subtask and the task scope is shutdown then the implementation doesn't currently force the owner to join when it's the first fork in a new round of forking. The result is that broken code calling Subtask::get throws ISE with the wrong exception message.
> 
> The existing test for this API is updated to remove a duplicate test and also fix two tasks that didn't call join.

Alan Bateman has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - Adjust exception messages
 - Merge
 - Merge
 - Merge
 - Initial commit

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/15076/files
  - new: https://git.openjdk.org/jdk/pull/15076/files/f88590bf..c2e9024c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15076&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15076&range=00-01

  Stats: 2789 lines in 110 files changed: 2488 ins; 65 del; 236 mod
  Patch: https://git.openjdk.org/jdk/pull/15076.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15076/head:pull/15076

PR: https://git.openjdk.org/jdk/pull/15076