Log unservicable response errors on HTTPServer

[avinashkamat48-design]

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).
✅ 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