code: public <T extends Exception> Javalin wsException(@NotNull Class<T> exceptionClass, @NotNull WsExceptionHandler<? super T> exceptionHandler) {
          wsServlet.getWsExceptionMapper().getHandlers().put(exceptionClass, (WsExceptionHandler<Exception>) exceptionHandler);
          return this;
      }
docs: Adds a WebSocket exception mapper to the instance.
  
  @see <a href="https://javalin.io/documentation#exception-mapping">Exception mapping in docs</a>