[foreign-memaccess+abi] RFR: Improve exception messages and simplify LayoutPath

[Per Minborg]

This PR proposes to improve the exception messages in the internal `LayoutPath` class and also do some code simplifications.

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Commit messages:
 - Revert more renaming
 - Revert variable renamings
 - Fix tests
 - Merge branch 'foreign-memaccess+abi' into layoutpath-improve
 - Improve messages and simplify LayoutPath

Changes: https://git.openjdk.org/panama-foreign/pull/886/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=886&range=00
  Stats: 30 lines in 3 files changed: 4 ins; 7 del; 19 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/886.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/886/head:pull/886

PR: https://git.openjdk.org/panama-foreign/pull/886