use AddError everywhere (#2372)

plugins/inputs/zookeeper/zookeeper.go

@@ -47,9 +47,7 @@ func (z *Zookeeper) Gather(acc telegraf.Accumulator) error {
 	}
 
 	for _, serverAddress := range z.Servers {
-		if err := z.gatherServer(serverAddress, acc); err != nil {
-			return err
-		}
+		acc.AddError(z.gatherServer(serverAddress, acc))
 	}
 	return nil
 }

plugins/inputs/zookeeper/zookeeper_test.go

@@ -19,8 +19,7 @@ func TestZookeeperGeneratesMetrics(t *testing.T) {
 
 	var acc testutil.Accumulator
 
-	err := z.Gather(&acc)
-	require.NoError(t, err)
+	require.NoError(t, acc.GatherError(z.Gather))
 
 	intMetrics := []string{
 		"avg_latency",