[crac] RFR: Minor code cleanup and improvements [v2]

[Radim Vansa]

> Extracted non-essential changes from other PR.

Radim Vansa has updated the pull request incrementally with one additional commit since the last revision:

  Use Combiner exception class for aggregate-only exceptions

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Changes:
  - all: https://git.openjdk.org/crac/pull/64/files
  - new: https://git.openjdk.org/crac/pull/64/files/a811566e..64e8ac66

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=crac&pr=64&range=01
 - incr: https://webrevs.openjdk.org/?repo=crac&pr=64&range=00-01

  Stats: 97 lines in 9 files changed: 69 ins; 4 del; 24 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