Surface a more detailed error message if user doesn't have server_private_key configured

[Leanngove]

Related issue: Resolves #43097

Checklist for submitter

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

• Input data is properly validated, SELECT * is avoided, SQL injection is prevented (using placeholders for values in statements), JS inline code is prevented especially for url redirects, and untrusted data interpolated into shell scripts/commands is validated against shell metacharacters.

• Timeouts are implemented and retries are limited to avoid infinite loops

• If paths of existing endpoints are modified without backwards compatibility, checked the frontend/CLI for any necessary changes

Testing

• Added/updated automated tests

• Where appropriate, automated tests simulate multiple hosts and test for host isolation (updates to one hosts's records do not affect another)

• QA'd all new/changed functionality manually

For unreleased bug fixes in a release candidate, one of:

• Confirmed that the fix is not expected to adversely impact load test results
• Alerted the release DRI if additional load testing is needed

Database migrations

• Checked schema for all modified table for columns that will auto-update timestamps during migration.
• Confirmed that updating the timestamps is acceptable, and will not cause unwanted side effects.
• Ensured the correct collation is explicitly set for character columns (COLLATE utf8mb4_unicode_ci).

New Fleet configuration settings

• Setting(s) is/are explicitly excluded from GitOps

If you didn't check the box above, follow this checklist for GitOps-enabled settings:

• Verified that the setting is exported via fleetctl generate-gitops
• Verified the setting is documented in a separate PR to the GitOps documentation
• Verified that the setting is cleared on the server if it is not supplied in a YAML file (or that it is documented as being optional)
• Verified that any relevant UI is disabled when GitOps mode is enabled

fleetd/orbit/Fleet Desktop

• Verified compatibility with the latest released version of Fleet (see Must rule)
• If the change applies to only one platform, confirmed that runtime.GOOS is used as needed to isolate changes
• Verified that fleetd runs on macOS, Linux and Windows
• Verified auto-update works from the released version of component to the new version (see tools/tuf/test)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.97%. Comparing base (3e4cf63) to head (2865dcf).
⚠️ Report is 197 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #49238      +/-   ##
==========================================
- Coverage   68.05%   67.97%   -0.09%     
==========================================
  Files        3684     3706      +22     
  Lines      234062   238599    +4537     
  Branches    12277    12277              
==========================================
+ Hits       159294   162187    +2893     
- Misses      60454    61628    +1174     
- Partials    14314    14784     +470     

Flag	Coverage Δ
backend	69.56% <100.00%> (-0.14%)	⬇️

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