Throw exception if setting during dynamic settings update isn't recognized (#26569)

Closes #25607

core/src/main/java/org/elasticsearch/common/settings/AbstractScopedSettings.java

@@ -495,6 +495,8 @@ public abstract class AbstractScopedSettings extends AbstractComponent {
                 // we don't validate if there is any dynamic setting with that prefix yet we could do in the future
                 toRemove.add(entry.getKey());
                 // we don't set changed here it's set after we apply deletes below if something actually changed
+            } else if (get(entry.getKey()) == null) {
+                throw new IllegalArgumentException(type + " setting [" + entry.getKey() + "], not recognized");
             } else if (entry.getValue() != null && canUpdate.test(entry.getKey())) {
                 validate(entry.getKey(), toApply);
                 settingsBuilder.put(entry.getKey(), entry.getValue());

core/src/test/java/org/elasticsearch/cluster/settings/ClusterSettingsIT.java

@@ -25,7 +25,6 @@ import org.elasticsearch.action.admin.cluster.settings.ClusterUpdateSettingsResp
 import org.elasticsearch.cluster.metadata.MetaData;
 import org.elasticsearch.cluster.routing.allocation.decider.EnableAllocationDecider;
 import org.elasticsearch.common.logging.ESLoggerFactory;
-import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.unit.ByteSizeUnit;
 import org.elasticsearch.discovery.Discovery;
@@ -33,10 +32,8 @@ import org.elasticsearch.discovery.DiscoverySettings;
 import org.elasticsearch.discovery.zen.ZenDiscovery;
 import org.elasticsearch.indices.recovery.RecoverySettings;
 import org.elasticsearch.test.ESIntegTestCase;
-import org.elasticsearch.test.ESIntegTestCase.ClusterScope;
 import org.junit.After;
 
-import static org.elasticsearch.test.ESIntegTestCase.Scope.TEST;
 import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertAcked;
 import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertBlocked;
 import static org.hamcrest.Matchers.containsString;
@@ -63,7 +60,7 @@ public class ClusterSettingsIT extends ESIntegTestCase {
                     .get();
             fail("bogus value");
         } catch (IllegalArgumentException ex) {
-            assertEquals(ex.getMessage(), "transient setting [no_idea_what_you_are_talking_about], not dynamically updateable");
+            assertEquals("transient setting [no_idea_what_you_are_talking_about], not recognized", ex.getMessage());
         }
     }