Provide a better error message when asynchronous operations are not supported by a filter or servlet. Patch provided by Romain Manni-Bucau. Issue was reported via users at tomcat.a.o.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1601796 13f79535-47bb-0310-9956-ffa450edef68