code: public <E extends Exception> RestHandlerExceptionResolverBuilder addHandler(
              Class<? extends E> exceptionClass, RestExceptionHandler<E, ?> exceptionHandler) {
  
          exceptionHandlers.put(exceptionClass, exceptionHandler);
          return this;
      }
docs: Registers the given exception handler for the specified exception type. This handler will be
  also used for all the exception subtypes, when no more specific mapping is found.
  
  @param exceptionClass The exception type handled by the given handler.
  @param exceptionHandler An instance of the exception handler for the specified exception
  type or its subtypes.