Error reporting and multiuser handling improvements.
When someone lands, log them if they're logged in, before making a new login attempt. Also, clear their session messages. This stops messages and potentially data for one user carrying over to another user when two people SSO from the same computer. Removed a catch (Exception) block, as now the code called in the throw block should always be throwing the correct exceptions. This will improve error reporting for SSO related errors quite significantly. Also throw some better exceptions if land.php fails to authenticate anyone.