etcdserver: handle proposal error when processing internal raft request

[nolouch]

after #9137 merged, may need to handle the proposal error in etcdserver.

This will be used to fix issue: #8975

[xiang90]

lgtm. can you fix CI failure?

[gyuho]

@nolouch Could you rebase from current master? Then, this should be able to pass CIs.

I just rebased. Merging in CI greens. Thanks!

[gyuho]

I've pushed another commit fixing a minor typo with changelog. All test suites passed including functional test https://semaphoreci.com/coreos/etcd/branches/pull-request-9549/builds/6. Merging.

Thanks.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@89c58b6). Click here to learn what that means.
The diff coverage is 20%.

@@            Coverage Diff            @@
##             master    #9549   +/-   ##
=========================================
  Coverage          ?   72.27%           
=========================================
  Files             ?      368           
  Lines             ?    31339           
  Branches          ?        0           
=========================================
  Hits              ?    22649           
  Misses            ?     7041           
  Partials          ?     1649

Impacted Files	Coverage Δ
etcdserver/v3_server.go	79.89% <20%> (ø)

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