SPIGOT-6642: Throw better message if plugin.yml is empty

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

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

@@ -1258,7 +1258,7 @@ public final class PluginDescriptionFile {
         if (object instanceof Map) {
             return (Map<?, ?>) object;
         }
-        throw new InvalidDescriptionException(object + " is not properly structured.");
+        throw new InvalidDescriptionException("Plugin description file is empty or not properly structured. Is " + object + "but should be a map.");
     }
 
     /**