Improve warning if spawner cannot acquire filelock (backport #2970)

[mergify]

Error logs in the CI are like

2026-01-20T03:49:35.9124785Z     [spawner-5] [WARN] [1768880313.776779189] [ros2_control_controller_spawner_fts_broadcaster]: Attempt 2 failed. Retrying in 3 seconds...
2026-01-20T03:49:35.9125179Z     [spawner-3] [WARN] [1768880313.776965745] [ros2_control_controller_spawner_joint_state_broadcaster]: Attempt 2 failed. Retrying in 3 seconds...

without any notice that the filelock is the problem here, not the service calls.

---

This is an automatic backport of pull request #2970 done by Mergify.

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.30%. Comparing base (9a0024d) to head (1586cfa).
⚠️ Report is 1 commits behind head on jazzy.

Files with missing lines	Patch %	Lines
controller_manager/controller_manager/spawner.py	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            jazzy    #2972      +/-   ##
==========================================
- Coverage   89.32%   89.30%   -0.02%     
==========================================
  Files         157      157              
  Lines       18675    18678       +3     
  Branches     1493     1494       +1     
==========================================
  Hits        16681    16681              
- Misses       1386     1390       +4     
+ Partials      608      607       -1     

Flag	Coverage Δ
unittests	89.30% <40.00%> (-0.02%)	⬇️

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

Files with missing lines	Coverage Δ
controller_manager/controller_manager/spawner.py	72.57% <40.00%> (-1.12%)	⬇️

... and 2 files with indirect coverage changes

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