[Twisted-web] small bug LivePage in 0.5

[Phil Mayers]

Accessing this page causes the server to spin at 100% CPU usage:

class WrongBaseClass(rend.Page):
   docFactory = loaders.stan(
     html[
       head[
         livepage.glue,
       ],
       body[
         p["blah"],
       ],
     ])

...whilst it's clearly an error, a helpful diagnostic message would have 
saved me hours of debugging including ripping out a perfectly good HTTP 
basic auth base class and implementing guard (talking of which)