Better error messages when using built-in labels

[juan-fdz-hawa]

Related issue: Resolves #32776

Checklist for submitter

If some of the following don't apply, delete the relevant line.

• Changes file added for user-visible changes in changes/, orbit/changes/ or ee/fleetd-chrome/changes.
  See Changes files for more information.

Testing

• Added/updated automated tests
• QA'd all new/changed functionality manually

[codecov]

Codecov Report

❌ Patch coverage is 62.96296% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.03%. Comparing base (ba4964d) to head (b87ef83).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/fleetctl/fleetctl/gitops.go	52.38%	7 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #34739   +/-   ##
=======================================
  Coverage   66.02%   66.03%           
=======================================
  Files        2065     2065           
  Lines      172531   172622   +91     
  Branches     6977     6977           
=======================================
+ Hits       113916   113990   +74     
- Misses      48106    48108    +2     
- Partials    10509    10524   +15     

Flag	Coverage Δ
backend	67.66% <62.96%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.