RFR: JDK-8306578: Report error if no label given in @see and {@link} when no default is available

[Hannes Wallnöfer]

Please review a trivial change to generate an error instead of a warning when no label is provided with a reference to a user-provided anchor.

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Commit messages:
 - JDK-8306578: Report error if no label given in @see and {@link} when no default is available

Changes: https://git.openjdk.org/jdk/pull/13720/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13720&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306578
  Stats: 6 lines in 2 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/13720.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13720/head:pull/13720

PR: https://git.openjdk.org/jdk/pull/13720