[Twisted-web] patch to guard

[Simon]

Posted a patch [1] to the Nevow tracker regarding guard.SessionWrapper.
I leave to those with better knowledge of this to decide if it's good
enough for a commit.

The need for the patch comes from that I want to be able to
differentiate between incorrect logins and logins denied due to, for
example, too many failed attempts. 
The patch modifies SessionWrapper.ebLoginError to trap LoginFailed
instead of 
UnauthorizedLogin and therefor catches all the subclasses of LoginFailed
(including UnauthorizedLogin and LoginDenied). 
The patch also let's you decide the message that will be appended to the
login-failure argument of the URL by taking the text representation of
the failure.

[1] http://divmod.org/users/roundup.twistd/nevow/issue166

/Simon