MINOR: Improve group metadata unknown key version exception message (#7006)

The patch clarifies the exception message for unknown key versions when loading from the group metadata topic. The patch also makes a trivial change in `KafkaAdminClient` to use `Map.computeIfAbsent`. 

Reviewers: Viktor Somogyi <viktorsomogyi@gmail.com>, Jason Gustafson <jason@confluent.io>

clients/src/main/java/org/apache/kafka/clients/admin/KafkaAdminClient.java

@@ -273,12 +273,7 @@ public class KafkaAdminClient extends AdminClient {
      * @return      The list value.
      */
     static <K, V> List<V> getOrCreateListValue(Map<K, List<V>> map, K key) {
-        List<V> list = map.get(key);
-        if (list != null)
-            return list;
-        list = new LinkedList<>();
-        map.put(key, list);
-        return list;
+        return map.computeIfAbsent(key, k -> new LinkedList<>());
     }
 
     /**

core/src/main/scala/kafka/coordinator/group/GroupMetadataManager.scala

@@ -1078,7 +1078,7 @@ object GroupMetadataManager {
     val schemaOpt = MESSAGE_TYPE_SCHEMAS.get(version)
     schemaOpt match {
       case Some(schema) => schema
-      case _ => throw new KafkaException("Unknown offset schema version " + version)
+      case _ => throw new KafkaException("Unknown message key schema version " + version)
     }
   }