RFR: 8347491: IllegalArgumentationException thrown by ThreadPoolExecutor doesn't have a useful message

[Viktor Klang]

Seems sensible to improve this given that it can be tricky to figure out which parameter caused the exception to be thrown.

-------------

Commit messages:
 - Addresses IAE and NPE messages for ThreadPoolExecutor

Changes: https://git.openjdk.org/jdk/pull/23081/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23081&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347491
  Stats: 25 lines in 1 file changed: 6 ins; 5 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/23081.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23081/head:pull/23081

PR: https://git.openjdk.org/jdk/pull/23081