Description of problem:
The LdapGroupSearcher code will fail if it tries to lookup a group that 
does not exist on the local ldap server.

This can happen when the ldap systems are configured as trusted domains.  
Even though the security-realm is not configured to use the trusted domain
(it is configured to only look at a single ldap server), the 
user's entry on one ldap server could point at a group that exists on 
the other (trusted) ldap server.

The LdapGroupSearcher code attempts to lookup this role and it fails.  This 
failure is sent back to the http server which results in an HTTP 500 error
and leaves the user with no way to authenticate/login.

There is currently not a way to tell the group searcher code to ignore the 
group/role that cannot be found.


How reproducible:
Create a user in ldap that has a "bogus" group.  Log into the admin console as that user.  Once the LdapGroupSearcher code looks up the user, it will fail when it attempts to lookup the "bogus" group.