comment to explain what happens next.

src/zope/server/http/wsgihttpserver.py

@@ -83,6 +83,10 @@ def start_response(status, headers, exc_info=None):
             if exc_info:
                 try:
                     if task.wroteResponseHeader():
+                        # higher levels will catch and handle raised exception:
+                        # 1. "service" method in httptask.py
+                        # 2. "service" method in severchannelbase.py
+                        # 3. "handlerThread" method in taskthreads.py
                         raise exc_info[0], exc_info[1], exc_info[2]
                     else:
                         # As per WSGI spec existing headers must be cleared