Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=56739
If an application handles an error on an application thread during asynchronous processing by calling HttpServletResponse.sendError(), then ensure that the application is given an opportunity to report that error via an appropriate application defined error page if one is configured.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1616441 13f79535-47bb-0310-9956-ffa450edef68