Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=57252
Provide application configured error pages with a chance to handle an
async error before the built-in error reporting.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1643210 13f79535-47bb-0310-9956-ffa450edef68