opt: eliminate error logs when the service exits normally

[panjf2000]

Fixes #497

[codecov]

Codecov Report

Patch coverage: 60.00% and project coverage change: -0.41% ⚠️

Comparison is base (3f217f9) 77.08% compared to head (9fcb6a2) 76.68%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #500      +/-   ##
==========================================
- Coverage   77.08%   76.68%   -0.41%     
==========================================
  Files          25       25              
  Lines        2457     2466       +9     
==========================================
- Hits         1894     1891       -3     
- Misses        429      435       +6     
- Partials      134      140       +6     

Flag	Coverage Δ
unittests	76.68% <60.00%> (-0.41%)	⬇️

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

Files Changed	Coverage Δ
engine_windows.go	89.47% <33.33%> (-6.32%)	⬇️
acceptor_windows.go	88.23% <62.50%> (-11.77%)	⬇️
listener_windows.go	77.77% <100.00%> (-0.80%)	⬇️

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