opt: eliminate error logs when the service exits normally

[panjf2000]

Fixes #497

[codecov[bot]]

Codecov Report

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

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/panjf2000/gnet/pull/500/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/panjf2000/gnet/pull/500/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan) | `76.68% <60.00%> (-0.41%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files Changed](https://app.codecov.io/gh/panjf2000/gnet/pull/500?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan) | Coverage Δ | | |---|---|---| | [engine\_windows.go](https://app.codecov.io/gh/panjf2000/gnet/pull/500?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan#diff-ZW5naW5lX3dpbmRvd3MuZ28=) | `89.47% <33.33%> (-6.32%)` | :arrow_down: | | [acceptor\_windows.go](https://app.codecov.io/gh/panjf2000/gnet/pull/500?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan#diff-YWNjZXB0b3Jfd2luZG93cy5nbw==) | `88.23% <62.50%> (-11.77%)` | :arrow_down: | | [listener\_windows.go](https://app.codecov.io/gh/panjf2000/gnet/pull/500?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan#diff-bGlzdGVuZXJfd2luZG93cy5nbw==) | `77.77% <100.00%> (-0.80%)` | :arrow_down: |

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