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