tools: Handle wporg error response in stable-tag.sh

[anomiex]

Proposed changes:

If the WordPress.org plugin API returns an error, we should report that and exit instead of trying to continue processing only to fail with something cryptic later on.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

p1714511067967399/1714502372.796139-slack-C01U2KGS2PQ

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• For the moment you can try running tools/stable-tag.sh plugins/automattic-for-agencies-client, as since the plugin is still in review the API returns a "Plugin not found" error.

[github-actions]

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