RFR: JDK-8306578: Report error if no label given in @see and {@link} when no default is available [v2]

[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.

Hannes Wallnöfer has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - Merge branch 'master' into JDK-8306578
 - JDK-8306578: Report error if no label given in @see and {@link} when no default is available

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Changes: https://git.openjdk.org/jdk/pull/13720/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13720&range=01
  Stats: 5 lines in 2 files changed: 0 ins; 0 del; 5 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