[issue 490] Add error log when schema encode failed. (#571)

* Add error log when schema encode failed.

* format code
diff --git a/pulsar/producer_partition.go b/pulsar/producer_partition.go
index 7e83bfa..abec4fc 100644
--- a/pulsar/producer_partition.go
+++ b/pulsar/producer_partition.go
@@ -356,6 +356,7 @@
 	if p.options.Schema != nil {
 		schemaPayload, err = p.options.Schema.Encode(msg.Value)
 		if err != nil {
+			p.log.WithError(err).Errorf("Schema encode message failed %s", msg.Value)
 			return
 		}
 	}