Make ErrorPageFilter.getDescription protected

Make `ErrorPageFilter#getDescription` `protected` instead of `private`
to be able to customize the details for the request logged in case of
an error.

Fixes spring-projectsgh-7380
Closes spring-projectsgh-7393

spring-boot/src/main/java/org/springframework/boot/web/support/ErrorPageFilter.java

@@ -185,7 +185,7 @@ private void forwardToErrorPage(String path, HttpServletRequest request,
 		request.getRequestDispatcher(path).forward(request, response);
 	}
 
-	private String getDescription(HttpServletRequest request) {
+	protected String getDescription(HttpServletRequest request) {
 		return "[" + request.getServletPath()
 				+ (request.getPathInfo() == null ? "" : request.getPathInfo()) + "]";
 	}