etcdserver/api/rafthttp: clarify "became inactive" warning

[gyuho]

"became inactive" is not a clear warning, when what's actually happening is failure to send HTTP requests.

[codecov-io]

Codecov Report

Merging #10024 into master will decrease coverage by 0.23%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #10024      +/-   ##
==========================================
- Coverage   69.37%   69.13%   -0.24%     
==========================================
  Files         386      386              
  Lines       36057    36060       +3     
==========================================
- Hits        25013    24930      -83     
- Misses       9243     9338      +95     
+ Partials     1801     1792       -9

Impacted Files	Coverage Δ
etcdserver/api/rafthttp/peer_status.go	89.47% <100%> (ø)	⬆️
etcdserver/api/v3rpc/interceptor.go	39.47% <0%> (-57.85%)	⬇️
clientv3/balancer/grpc1.7-health.go	46.22% <0%> (-12.8%)	⬇️
proxy/grpcproxy/register.go	69.44% <0%> (-11.12%)	⬇️
pkg/netutil/netutil.go	63.11% <0%> (-6.56%)	⬇️
proxy/grpcproxy/watcher.go	85.71% <0%> (-4.09%)	⬇️
proxy/httpproxy/director.go	80% <0%> (-2.86%)	⬇️
clientv3/concurrency/election.go	79.52% <0%> (-2.37%)	⬇️
etcdserver/api/v3rpc/watch.go	73.52% <0%> (-2.29%)	⬇️
integration/bridge.go	70.22% <0%> (-1.53%)	⬇️
... and 17 more

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[spzala]

lgtm