Cherry-pick #24281 to 7.x: [Elastic Agent] Fix #24273 - Fix SIGSEGV due to missing err check

[blakerouse]

Cherry-pick of PR #24281 to 7.x branch. Original message:

What does this PR do?

Fixes a missing checkout for an error when performing bootstrap of Fleet Server.

Why is it important?

So proper error is reported and to not cause a SIGSEGV.

Checklist

• 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.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Closes Elastic Agent crashes while trying to start the agent with Fleet Server #24273

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[elasticmachine]

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Build stats

• Build Cause: Pull request #24282 opened

• Start Time: 2021-03-01T21:24:39.933+0000

• Duration: 52 min 18 sec

• Commit: d80cd91

Test stats 🧪

Test	Results
Failed	0
Passed	6486
Skipped	24
Total	6510

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Test stats 🧪

Test	Results
Failed	0
Passed	6486
Skipped	24
Total	6510

[michalpristas]

good catch