Nice ImportError raised if you try to use twisted without having it,

lib/python/Zope2/Startup/__init__.py

@@ -117,7 +117,7 @@ class ZopeStarter:
             config = getConfiguration()
             if config.twisted_servers:
                 if not _use_twisted:
-                    raise ValueError, "You do not have twisted installed."
+                    raise ImportError("You do not have twisted installed.")
                 twisted.internet.reactor.run()
             else:
                 import ZServer

lib/python/Zope2/Startup/datatypes.py

@@ -345,4 +345,5 @@ try:
         pass
 except ImportError:
     class TwistedServerFactory:
-        pass
+        def __init__(self, section):
+            raise ImportError("You do not have twisted installed.")