Fix error logging when sending 503 response fails

[avinashkamat48]

Summary

• replace the invalid self.server.error_log call in HTTPServer._serve_unservicable()
• add a focused regression test for exceptions raised while sending the polite 503 response

Verification

• python -m pytest cheroot/test/test_server.py::test_serve_unservicable_logs_simple_response_errors -q (test passed; command exited nonzero because single-test coverage is below the repo-wide fail-under threshold)
• git diff --check

Closes #797

[read-the-docs-community]

Documentation build overview

📚 cheroot | 🛠️ Build #33095683 | 📁 Comparing a8a068b against latest (2ffb0ba)

  🔍 Preview build  

7 files changed · ± 7 modified

± Modified

• genindex/index.html
• history/index.html
• pkg/cheroot/index.html
• pkg/cheroot.server/index.html
• pkg/cheroot.ssl.pyopenssl/index.html
• pkg/cheroot.test/index.html
• pkg/cheroot.test.test_server/index.html

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.28%. Comparing base (2ffb0ba) to head (a8a068b).
⚠️ Report is 5 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #825      +/-   ##
==========================================
- Coverage   78.32%   78.28%   -0.04%     
==========================================
  Files          41       41              
  Lines        4788     4817      +29     
  Branches      547      547              
==========================================
+ Hits         3750     3771      +21     
- Misses        900      907       +7     
- Partials      138      139       +1     

[julianz-]

@avinashkamat48 Thanks for this PR. There are a few housekeeping issues here:

1. The function _serve_unservicable() has a spelling mistake (should be unserviceable) which is not your fault but I have changed the PR title to avoid repeating the same error. I am not suggesting we change the name in the code in this PR - just that we don't repeat its use in comments and titles if we can avoid it.

2. There is a description template with a checklist for PRs which @webknjaz would like us to use.

3. You need to add a docs/changelog-fragments.d/825.bugfix.rst describing the bug fix here in the appropriate towncrier style - see docs/changelog-fragments.d/README.rst.

4. The commit is missing a body. You can see this in the CI log:

https://github.com/cherrypy/cheroot/actions/runs/27353939982/job/80822995068?pr=825

Commit 5ef1cb8897:
3: B6 Body message is missing

5. git diff --check in your "Verification" section would only check for white space issues that pre-commit already checks for so this is not a meaningful test.