Clarify message on resize scaling executor queues

This commit clarifies an error message that is produced when an attempt
is made to resize the backing queue for a scaling executor. As this
queue is unbounded, resizing the backing queue does not make sense. The
clarification here is to specify that this restriction is because the
executor is a scaling executor.

core/src/main/java/org/elasticsearch/threadpool/ThreadPool.java

@@ -525,7 +525,7 @@ public class ThreadPool extends AbstractComponent implements Closeable {
             int defaultSize = defaultSettings.getAsInt("size", EsExecutors.boundedNumberOfProcessors(settings));
             final Integer queueSize = settings.getAsInt("queue_size", defaultSettings.getAsInt("queue_size", null));
             if (queueSize != null) {
-                throw new IllegalArgumentException("thread pool [" + name + "] can not have its queue re-sized but was [" + queueSize + "]");
+                throw new IllegalArgumentException("thread pool [" + name + "] of type scaling can not have its queue re-sized but was [" + queueSize + "]");
             }
             if (previousExecutorHolder != null) {
                 if (ThreadPoolType.SCALING == previousInfo.getThreadPoolType()) {

core/src/test/java/org/elasticsearch/threadpool/ScalingThreadPoolTests.java

@@ -212,7 +212,8 @@ public class ScalingThreadPoolTests extends ESThreadPoolTestCase {
                     IllegalArgumentException.class,
                     () -> clusterSettings.applySettings(settings("threadpool." + threadPoolName + ".queue_size", size)));
             assertThat(e, hasToString(
-                    "java.lang.IllegalArgumentException: thread pool [" + threadPoolName + "] can not have its queue re-sized but was [" +
+                    "java.lang.IllegalArgumentException: thread pool [" + threadPoolName +
+                        "] of type scaling can not have its queue re-sized but was [" +
                             size + "]"));
         });
     }