RFR: 8308764: Reporting errors from create_vm may crash

[David Holmes]

There are more details in the bug report but the basic fix is to not delete the main_thread so that it is available to report the exception that caused VM initialization to fail. Added some additional comments too.

Testing:
 - basic fault injection to test non-exception exit paths both before and after Universe:;is_fully_initialized() which exercises the failing use of JavaThread::current()
 -  tiers 1-3 sanity testing

Thanks.

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Commit messages:
 - Fixed typo
 - 8308764: Reporting errors from create_vm may crash

Changes: https://git.openjdk.org/jdk/pull/14238/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14238&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308764
  Stats: 8 lines in 2 files changed: 7 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14238.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14238/head:pull/14238

PR: https://git.openjdk.org/jdk/pull/14238