KAFKA-7921: log at error level for missing source topic (#6262)

This condition is a fatal error, so error level is warranted, to provide more context on why Streams shuts down.

Reviewers: Matthias J. Sax <matthias@confluent.io>, Guozhang Wang <guozhang@confluent.io>

streams/src/main/java/org/apache/kafka/streams/processor/internals/StreamThread.java

@@ -267,7 +267,7 @@ public class StreamThread extends Thread {
                 taskManager.suspendedStandbyTaskIds());
 
             if (streamThread.assignmentErrorCode.get() == StreamsPartitionAssignor.Error.INCOMPLETE_SOURCE_TOPIC_METADATA.code()) {
-                log.debug("Received error code {} - shutdown", streamThread.assignmentErrorCode.get());
+                log.error("Received error code {} - shutdown", streamThread.assignmentErrorCode.get());
                 streamThread.shutdown();
                 streamThread.setStateListener(null);
                 return;

streams/src/main/java/org/apache/kafka/streams/processor/internals/StreamsPartitionAssignor.java

@@ -463,6 +463,8 @@ public class StreamsPartitionAssignor implements PartitionAssignor, Configurable
             for (final String topic : topicsInfo.sourceTopics) {
                 if (!topicsInfo.repartitionSourceTopics.keySet().contains(topic) &&
                     !metadata.topics().contains(topic)) {
+                    log.error("Missing source topic {} durign assignment. Returning error {}.",
+                              topic, Error.INCOMPLETE_SOURCE_TOPIC_METADATA.name());
                     return errorAssignment(clientsMetadata, topic, Error.INCOMPLETE_SOURCE_TOPIC_METADATA.code);
                 }
             }