#512 External Auth returns blank page when missing php modules

[Matt Brown]

This was reported as Debian Bug #331709, see
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=331709

If a user sets USER_AUTH_ORDER to one of the external
auth methods that requires extra PHP modules (such as
IMAP or LDAP) but neglects to install the PHP modules
onto their system, authentication fails nastily.

After typing in the username and password, a blank page
(0 byte response) is returned by PHPwiki.

It is expected that a normal error message should be
displayed saying that the user could not be logged in.