GH-774: Rethrow e in KT.executeInTransaction()

Fixes #774

**cherry-pick to 2.1.x, 2.0.x, 1.3.x**

Author: garyrussell

spring-kafka/src/main/java/org/springframework/kafka/core/KafkaTemplate.java

@@ -285,17 +285,15 @@ public <T> T executeInTransaction(OperationsCallback<K, V, T> callback) {
 			finally {
 				this.producers.remove();
 				closeProducer(producer, false);
-				producer = null;
 			}
+			throw e;
 		}
-		if (producer != null) {
-			try {
-				producer.commitTransaction();
-			}
-			finally {
-				closeProducer(producer, false);
-				this.producers.remove();
-			}
+		try {
+			producer.commitTransaction();
+		}
+		finally {
+			this.producers.remove();
+			closeProducer(producer, false);
 		}
 		return result;
 	}