RFR: 8371440: jpackage should exit with an error if it finds multiple matching signing certificates

[Alexey Semenyuk]

jpackage will exit with an error if the value of the `--mac-signing-key-user-name` option matches multiple certificates of the same type (app image or installer).

Previously, jpackage would emit an error that multiple certificates match the value of the `--mac-signing-key-user-name` option, pick the first matching certificate, and keep going.

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Commit messages:
 - 8371440: jpackage should exit with an error if it finds multiple matching signing certificates

Changes: https://git.openjdk.org/jdk/pull/28405/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28405&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371440
  Stats: 36 lines in 3 files changed: 16 ins; 7 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/28405.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28405/head:pull/28405

PR: https://git.openjdk.org/jdk/pull/28405