Add custom MarshalYAML() method to component to fix YAML marshaling of error values

[fearful-symmetry]

What does this PR do?

fixes #2940

This adds a MarshalYAML() method to the Component type that populates an ErrMsg value. This is needed to properly report any potential error values that might end up in a diagnostic dump; depending on the underlying type that's in the Err field, the YAML marshaller might just end up creating an empty {} value in the final YAML. I figured this approach, of creating a special ErrMsg field, was less invasive than coming up with some custom error wrapper that we would need to give special treatment.

Why is it important?

Errors should be properly reported in diagnostic dumps

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/fragments using the changelog tool
• I have added an integration test or an E2E test

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[mergify]

This pull request does not have a backport label. Could you fix it @fearful-symmetry? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 8./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elastic-sonarqube]

SonarQube Quality Gate

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

100.0% Coverage
0.0% Duplication

[blakerouse]

Looks good.

Thanks for the improvement!

[pierrehilbert]

@fearful-symmetry What would you think about backporting this to 8.11 branch?
cc @cmacknz

[cmacknz]

Good idea, added the backport label