RFR: 8338833: Error on reference not found for a snippet target

[Hannes Wallnöfer]

Please review a trivial fix to generate an error when a link in a snippet has an invalid target.

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Commit messages:
 - 8338833: Error on reference not found for a snippet target

Changes: https://git.openjdk.org/jdk/pull/25329/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25329&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338833
  Stats: 49 lines in 2 files changed: 46 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/25329.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25329/head:pull/25329

PR: https://git.openjdk.org/jdk/pull/25329