[foreign-memaccess+abi] RFR: Improve exception messages and simplify LayoutPath [v5]

[Per Minborg]

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

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Fix exception messages

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/886/files
  - new: https://git.openjdk.org/panama-foreign/pull/886/files/a4c26f69..ae904ec8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=886&range=04
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=886&range=03-04

  Stats: 26 lines in 3 files changed: 22 ins; 0 del; 4 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