refactor: make resonse error types validatable

src/protocol/errors/plugin-already-registered.ts

@@ -20,6 +20,8 @@ import {SCPluginMetaData} from '../routes/plugin-register';
  * An error that is returned when a plugin with the same name is already registered, to prevent two copies of a plugin
  * running at the same time.
  * This usually indicates that there is more than one instance a plugin running.
+ *
+ * @validatable
  */
 export class SCPluginAlreadyRegisteredErrorResponse extends SCError {
   /**