[8.19] (backport #12225) Report app lock errors correctly

[mergify]

What does this PR do?

Ensures we emit the underlying error if we fail to acquire the app lock. Before, we'd erroneously just put nil in there.

Why is it important?

We should always report the underlying error, as it helps with troubleshooting.

Checklist

• I have read and understood the pull request guidelines of this project.
• My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• [ ] I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• [ ] I have added an integration test or an E2E test

How to test this PR locally

Run two agents concurrently from the same data directory. You should get:

Error: failed to get app lock: another elastic-agent is already running

Before this change, it's:

Error: failed to get app lock: %!w(<nil>)

---

This is an automatic backport of pull request #12225 done by [Mergify](https://mergify.com).

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[mergify]

This pull request has not been merged yet. Could you please review and merge it @swiatekm? 🙏

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: bf87151

History

• 💔 Build #33555 failed b266337
• 💔 Build #33549 failed 03081ef
• 💔 Build #33495 failed 4d39568
• 💔 Build #33472 failed d196779
• 💔 Build #33448 failed 75f5491

cc @swiatekm