RFR: 8373253: Re-work InjectGCWorkerCreationFailure for future changes [v4]

[Leo Korinth]

> This PR slightly changes when we may `InjectGCWorkerCreationFailure`.  At the moment we wait until `is_init_completed()`. I am changing this to instead be after `_created_workers > 0`. The reason is that we might in the future, https://bugs.openjdk.org/browse/JDK-8367993,  create even more threads "on demand", and if so, we would fail VM creation if we inject worker creation failures after the VM is initiated but before we have created any of the worker threads.
> 
> This change should not change the current behaviour. But help future improvements.
> 
> I have tested this on `test/hotspot/jtreg/gc/stress/gcold/TestGCOldWithG1.java`, the only place where this flag is tested to my knowledge.

Leo Korinth has updated the pull request incrementally with four additional commits since the last revision:

 - Proposal by Stefan J
 - wip
 - Revert "8373253: Re-work InjectGCWorkerCreationFailure for future changes"
   
   This reverts commit d45ea8817ab2303b2decd8cbb2cd1bf5280aa181.
 - Revert "Fixup after comment from Ivan."
   
   This reverts commit 2aa8aa4b68027b62a8d4be1b86720fadfa48dda5.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28719/files
  - new: https://git.openjdk.org/jdk/pull/28719/files/11649b67..5703deb1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28719&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28719&range=02-03

  Stats: 25 lines in 2 files changed: 18 ins; 7 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28719.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28719/head:pull/28719

PR: https://git.openjdk.org/jdk/pull/28719