Add missing HTTP error pages on bits.wm.o

Currently, hitting a 404 on bits gives an error page containing
that annoying message, "Additionally, a 404 Not Found error was
encountered while trying to use an ErrorDocument to handle the
request." which, to me, says "you need to learn how to maintain
a server!" We can do better than that.

Change-Id: I1c6985f0faf131aa04f4552fd71689004f337924