How to make ServletFilter respond message display on the page?

I have write a java class that can intercept the page copy action. The atlassian-plugin.xml source code is as bellow:

intercept page copy action if there are too many children pages

however, in the class after I handle the logic, I use the method “sendError” of HttpServletResponse class to response some error message. For example: response.sendError(HttpServletResponse.SC_FORBIDDEN, “hello, there are something wrong, please contact someone”);
But these error messages are not displayed on the page.It is so confusing to me.
So how can these error messages be displayed on the page??