etcdserver: correct error log for strict reconfig checking

This commit fixes an error log caused by the strict reconfig checking
option.

Before:
14:21:38 etcd2 | 2015-11-05 14:21:38.870356 E | etcdhttp: got unexpected response error (etcdserver: re-configuration failed due to not enough started members)

After:
log
13:27:33 etcd2 | 2015-11-05 13:27:33.089364 E | etcdhttp: etcdserver: re-configuration failed due to not enough started members

The error is not an unexpected thing therefore the old message is
incorrect.

etcdserver/etcdhttp/http.go

@@ -59,7 +59,7 @@ func writeError(w http.ResponseWriter, r *http.Request, err error) {
 		}
 	default:
 		switch err {
-		case etcdserver.ErrTimeoutDueToLeaderFail, etcdserver.ErrTimeoutDueToConnectionLost:
+		case etcdserver.ErrTimeoutDueToLeaderFail, etcdserver.ErrTimeoutDueToConnectionLost, etcdserver.ErrNotEnoughStartedMembers:
 			plog.Error(err)
 		default:
 			plog.Errorf("got unexpected response error (%v)", err)