Improve error handling for an unhandled exception after the response has been committed. Tomcat will now attempt to:
- flush any unwritten response data to the client
- prevent further writes to the response
- close the connection

This means that the client should experience an unclean close that will enable them to differentiate (when chunked encoding is used) between an incomplete response that encountered an error and a complete response that did not (prior to this commit, Tomcat completed the request normally and - depending on the response data written to that point - it may not have been visible to the client that the response was incomplete at that point.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1600449 13f79535-47bb-0310-9956-ffa450edef68