[SPIGOT-950] Make handlerList requirement really obvious.

By: md_5 <git@md-5.net>

paper-api/src/main/java/org/bukkit/event/Event.java

@@ -5,6 +5,8 @@ import org.bukkit.plugin.PluginManager;
 /**
  * Represents an event.
  *
+ * All events require a static method named getHandlerList() which returns the same {@link HandlerList} as {@link #getHandlers()}.
+ *
  * @see PluginManager#callEvent(Event)
  * @see PluginManager#registerEvents(Listener,Plugin)
  */

paper-api/src/main/java/org/bukkit/plugin/SimplePluginManager.java

@@ -582,7 +582,7 @@ public final class SimplePluginManager implements PluginManager {
                     && Event.class.isAssignableFrom(clazz.getSuperclass())) {
                 return getRegistrationClass(clazz.getSuperclass().asSubclass(Event.class));
             } else {
-                throw new IllegalPluginAccessException("Unable to find handler list for event " + clazz.getName());
+                throw new IllegalPluginAccessException("Unable to find handler list for event " + clazz.getName() + ". Static getHandlerList method required!");
             }
         }
     }