Check if wgAuth is set correctly

If wgAuth isn't set correctly (or is overwritten by another Extension), show
and Error Message an exit.

Bug: 68962
Change-Id: Id0d33e96d5f00eee067e680e5e62b58b32caf498
Reviewed-on: https://gerrit.wikimedia.org/r/151444
Reviewed-by: Siebrand <siebrand@kitano.nl>
Tested-by: jenkins-bot