clientv3: update error handling godoc

[gyuho]

Fix #8709.

[xiang90]

lgtm

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@4ad8bd9). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #8936   +/-   ##
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  Coverage          ?   76.12%           
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  Files             ?      359           
  Lines             ?    29791           
  Branches          ?        0           
=========================================
  Hits              ?    22678           
  Misses            ?     5539           
  Partials          ?     1574

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