[crac] RFR: Use special class for exception aggregates [v5]

[Radim Vansa]

> Extracted non-essential changes from other PR.

Radim Vansa has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains six commits:

 - Merge branch 'crac' into code_cleanup
 - Remove the .Combined, always use the main exception as aggregate
 - Merge branch 'crac' into code_cleanup
 - Handle .Combined in Core
 - Use Combiner exception class for aggregate-only exceptions
 - Minor code cleanup and improvements

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Changes: https://git.openjdk.org/crac/pull/64/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=64&range=04
  Stats: 69 lines in 9 files changed: 9 ins; 39 del; 21 mod
  Patch: https://git.openjdk.org/crac/pull/64.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/64/head:pull/64

PR: https://git.openjdk.org/crac/pull/64