Update docs build to tolerate tags that have been pushed but not yet built and published

The v4.0.2 tag has been pushed but the resulting spring-boot-docs-4.0.2-root-aggregate-content.zip artifact has not yet been built and published. This breaks the docs build:

Run node run.js --no-checkout
  node run.js --no-checkout
  shell: /usr/bin/bash -e {0}
  env:
    ARTIFACTORY_USERNAME: ***
    ARTIFACTORY_PASSWORD: ***
    COMMERCIAL_REPO_USERNAME: 
    COMMERCIAL_REPO_PASSWORD: 
    COMMERCIAL_SNAPSHOT_REPO_URL: 
    COMMERCIAL_RELEASE_REPO_URL: 
    BUILD_REFNAME: 3.5.x
    BUILD_VERSION: 3.5.11-SNAPSHOT
Installing modules
Running Antora
$ npx antora antora-playbook.yml --stacktrace --ui-bundle-url https://github.com/spring-io/antora-ui-spring/releases/download/v0.4.25/ui-bundle.zip
partial-build-extension: version 3.5.11 not previously built; reverting to full build
[12:44:03.539] FATAL (antora): Unable to download 'https://repo.spring.io/release/org/springframework/boot/spring-boot-docs/4.0.2/spring-boot-docs-4.0.2-root-aggregate-content.zip' due to HTTP response code 404 ()
    Cause: HTTPError
        at download (/home/runner/work/spring-boot/spring-boot/node_modules/@springio/antora-zip-contents-collector-extension/lib/index.js:409:27)
        at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
        at async getZipFile (/home/runner/work/spring-boot/spring-boot/node_modules/@springio/antora-zip-contents-collector-extension/lib/index.js:292:20)
        at async doWithIncludes (/home/runner/work/spring-boot/spring-boot/node_modules/@springio/antora-zip-contents-collector-extension/lib/index.js:251:23)
        at async addContentAggregateIncludes (/home/runner/work/spring-boot/spring-boot/node_modules/@springio/antora-zip-contents-collector-extension/lib/index.js:115:7)
        at async GeneratorContext.contentAggregated (/home/runner/work/spring-boot/spring-boot/node_modules/@springio/antora-zip-contents-collector-extension/lib/index.js:50:11)
        at async GeneratorContext.notify (/home/runner/work/spring-boot/spring-boot/node_modules/@antora/site-generator/lib/generator-context.js:66:39)
        at async Promise.all (index 0)
        at async generateSite (/home/runner/work/spring-boot/spring-boot/node_modules/@antora/site-generator/lib/generate-site.js:25:5)
        at async Command.parseAsync (/home/runner/work/spring-boot/spring-boot/node_modules/commander/lib/command.js:1092:5)
Unexpected error

It would be better if it somehow ignored the tag instead.

Start with run.js and antora-playbook.yml, then reproduce the failure using node run.js --no-checkout. Trace how the partial-build-extension and Antora zip-content collection handle the missing spring-boot-docs-4.0.2 artifact. Done means the docs build skips tags whose artifacts are not yet published while continuing to include available content.