RFR: 7903877: jextract exception handling in downcall wrappers

[Nizar Benalla]

Can I please get a review for this patch to improve the exception handling in downcall wrappers, if the exception is reachable then throwing an exception is better than catching an exception and turning them into an `AssertionError`.

TIA

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Commit messages:
 - improve downcall wrapper exception handling

Changes: https://git.openjdk.org/jextract/pull/282/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=282&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903877
  Stats: 124 lines in 5 files changed: 120 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jextract/pull/282.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/282/head:pull/282

PR: https://git.openjdk.org/jextract/pull/282