RFR: 8284400: Improve XPath exception handling

[Joe Wang]

Address some insufficiency of error handling in the XPath implementation. Some cleanup where appropriate, without attempting to do too much as this is an component that hasn't had much changes for 15 years, a fairly stable application.

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Commit messages:
 - 8284400: Improve XPath exception handling

Changes: https://git.openjdk.java.net/jdk/pull/8910/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8910&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284400
  Stats: 861 lines in 11 files changed: 617 ins; 179 del; 65 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8910.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8910/head:pull/8910

PR: https://git.openjdk.java.net/jdk/pull/8910