[foreign-memaccess+abi] RFR: Improve exception messages and simplify LayoutPath [v2]

[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:

  Make the unmapper of MappedByteBuffer safe

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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/ec808083..b5df6021

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=886&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=886&range=00-01

  Stats: 33 lines in 1 file changed: 11 ins; 0 del; 22 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