Fail to start if plugin tries broken onModule

If a plugin declares `onModule(SomethingThatIsntAModule)` then refuse
to start. Before this commit we just logged a warning that flies by in
the console and is easy to miss. You can't miss refusing to start!

core/src/main/java/org/elasticsearch/plugins/PluginsService.java

@@ -44,6 +44,7 @@ import org.elasticsearch.index.IndexModule;
 import org.elasticsearch.script.NativeScriptFactory;
 import org.elasticsearch.script.ScriptContext;
 import org.elasticsearch.script.ScriptEngineService;
+import org.elasticsearch.script.ScriptModule;
 import org.elasticsearch.threadpool.ExecutorBuilder;
 
 import java.io.IOException;
@@ -206,8 +207,13 @@ public class PluginsService extends AbstractComponent {
                 }
                 Class moduleClass = method.getParameterTypes()[0];
                 if (!Module.class.isAssignableFrom(moduleClass)) {
-                    logger.warn("Plugin: {} implementing onModule by the type is not of Module type {}", pluginEntry.v1().getName(), moduleClass);
+                    if (moduleClass == ScriptModule.class) {
-                    continue;
+                        // This is still part of the Plugin class to point the user to the new implementation
+                        continue;
+                    }
+                    throw new RuntimeException(
+                            "Plugin: [" + pluginEntry.v1().getName() + "] implements onModule taking a parameter that isn't a Module ["
+                                    + moduleClass.getSimpleName() + "]");
                 }
                 list.add(new OnModuleReference(moduleClass, method));
             }

docs/reference/migration/migrate_5_0/plugins.asciidoc

@@ -117,3 +117,8 @@ via ES_JAVA_OPTS.
 
 The ability to specify a custom plugins path via `path.plugins` has
 been removed.
+
+==== ScriptPlugin
+
+Plugins that register custom scripts should implement `ScriptPlugin` and remove
+their `onModule(ScriptModule)` implementation.