rafthttp: stop masking errMemberRemoved in pipeline

It makes logic more straightforward and readable. Also, it makes the
handle method consistent with stream and snapshot sender.

rafthttp/pipeline.go

@@ -159,7 +159,6 @@ func (p *pipeline) post(data []byte) (err error) {
 		// always be stopped. So we use reportCriticalError to report it to errorc.
 		if err == errMemberRemoved {
 			reportCriticalError(err, p.errorc)
-			return nil
 		}
 		return err
 	}

rafthttp/transport_test.go

@@ -128,6 +128,7 @@ func TestTransportUpdate(t *testing.T) {
 func TestTransportErrorc(t *testing.T) {
 	errorc := make(chan error, 1)
 	tr := &Transport{
+		Raft:        &fakeRaft{},
 		LeaderStats: stats.NewLeaderStats(""),
 		ErrorC:      errorc,
 		streamRt:    newRespRoundTripper(http.StatusForbidden, nil),